Fix order of GREATEST for Oracle

As per https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions060.htm


Oracle uses the first value to cast the rest or the values.
So when the first value is a plain int, instead of doing the math,
it will cast the expression to int and continue with a potential 0.

Signed-off-by: Joas Schilling <coding@schilljs.com>