You need to change default query terminator([;]) to sth else than ; and you may need to add this separator and new line between blocks of code:

SqlFiddleDemo

DECLARE
   x NUMBER := 100;
BEGIN
   FOR i IN 1..10 LOOP
      IF MOD(i,2) = 0 THEN     
         INSERT INTO temp VALUES (i, x, 'i is even');
      ELSE
         INSERT INTO temp VALUES (i, x, 'i is odd');
      END IF;
      x := x + 100;
   END LOOP;
   COMMIT;
END;

What's up with that [ ; ] button under each panel?

This obscure little button determines how the queries in each of the panels get broken up before they are sent off to the database. This button pops open a dropdown that lists different "query terminators." Query terminators are used as a flag to indicate (when present at the end of a line) that the current statement has ended. The terminator does not get sent to the database; instead, it merely idicates how I should parse the text before I execute the query.

Oftentimes, you won't need to touch this button; the main value this feature will have is in defining stored procedures. This is because it is often the case that within a stored procedure's body definition, you might want to end a line with a semicolon (this is often the case). Since my default query terminator is also a semicolon, there is no obvious way for me to see that your stored procedure's semicolon isn't actually the end of the query. Left with the semicolon terminator, I would break up your procedure definition into incorrect parts, and errors would certainly result. Changing the query terminator to something other than a semicolon avoids this problem.

The procedure has to be ended with a slash; so you need to add that, and change the build-schema command separator from the default ; to / - and then change all of the other separators in your code from semicolons to slashes:

create table tab (
     id_tab integer not null,
     val1 integer,
     val2 integer,
     val3 integer,
     val4 integer,
     val5 integer,
     val6 integer,
     val7 integer,
     val8 integer,
     val9 integer,
     CONSTRAINT tab_pk PRIMARY KEY (id_tab)
)
/
  
create index val1_index on tab (val1)
/
create index val2_index on tab (val2)
/
create index val3_index on tab (val3)
/
create index val4_index on tab (val4)
/
create index val5_index on tab (val5)
/
create index val6_index on tab (val6)
/
create index val7_index on tab (val7)
/
create index val8_index on tab (val8)
/  
create index val9_index on tab (val9)
/
  
create procedure test1 as
begin
  for x in 1..1000
  loop
    insert into tab(id_tab, val1, val2, val3, val4, val5, val6, val7, val8, val9)
    values ((select nvl(max(id_tab), 0) + 1 from tab), 
            dbms_random.value(1,9), 
            dbms_random.value(1,9), 
            dbms_random.value(1,9), 
            dbms_random.value(1,9), 
            dbms_random.value(1,9), 
            dbms_random.value(1,9),
            dbms_random.value(1,9), 
            dbms_random.value(1,9), 
            dbms_random.value(1,9));
  end loop;
end;
/

As you need an anonymous block to call the procedure, you need to do the same thing - including changing the default separator - in the other section too:

begin
  test1;
end;
/

select * from tab
/

SQL Fiddle

As @Lieven correctly pointed out: There is a button below right where you can toggle the query terminator to /.

Working example at http://www.sqlfiddle.com/#!4/db36d/5

I've found the solution!
schema pane, line ending / both panes

CREATE TABLE user_1 (password varchar(10))
/
create procedure updateUsers
   ( new_p in varchar2)
as
begin
SAVEPOINT before;
  update user_1 set password = new_p;
  dbms_output.put_line(SQL%RowCount);
commit;
EXCEPTION WHEN OTHERS THEN DBMS_OUTPUT.PUT_LINE('Error: ' || SQLCODE || ' - ' || SQLERRM); 
end;
/

query pane

insert into user_1 values ('initial')/
select password from user_1/
update user_1 set password = 'from query'/
SELECT password from user_1/
begin
  updateUsers('new_pass') ;
end;
/
select password from user_1/

output

PASSWORD
initial

PASSWORD
from query

PASSWORD
new_pass

see also extension
http://sqlfiddle.com/#!4/b942cc/3

Add a / after the function definition.

https://dbfiddle.uk/?rdbms=oracle_18&fiddle=4031e203694674dbc764b5290dd93bdb

UPDATE: All seems to be working again. See the answer from Jake Feasel (the creator of SQL Fiddle).

To summarise info from the comments as an answer and bring up to date as of writing:

(Please comment if any of the above changes and I'll update the answer accordingly).

SQLFiddle Alternatives

1. http://rextester.com has options for MySQL, Oracle, PostgreSQL and SQL Server in the "Language" dropdown (as well as the ability to save work online in a similar way to SQLFiddle).
2. http://db-fiddle.com has options for MySQL 5.5 / 5.6 / 5.7 / 8.0, PostgreSQL 9.4 / 9.5 / 9.6 / 10.0 and SQLite 3.16 / 3.17 / 3.18.
3. https://dbfiddle.uk has options for MariaDB 10.2 / 10.3, MySQL 8.0, Oracle 11g Release 2, Postgres 8.4 / 9.4 / 9.5 / 9.6 / 10 / 11, SQL Server 2012 / 2014 / 2016 / 2017 / 2017 (Linux) and SQLite 3.8 / 3.16 (as well as the ability to save work online in a similar way to SQLFiddle). See further discussion here.
4. Suggested alternatives for Oracle are Oracle Application Express and Oracle Live SQL.
5. A MySQL alternative with more limited functionality is sqlize.com.
6. A clunky MySQL alternative is sqltest.net (if you can find your way around the adverts).
7. As a last resort, I've occasionally seen answers that use Stack Exchange Data Explorer to create custom queries (in SQL Server 2016) that aren't based on the built-in tables. [Disclaimer: I'm not sure it's really supposed to be used for this kind of purpose.]