Allow the 'gc_probability' option to be set to zero.

This is valid because PHP divides gc_divisor into gc_probability - it would only
be invalid to set gc_divisor to 0 (divide-by-zero), so this commit leaves the
same test for that option alone.
Merge pull request #4608 from gwis/allow-zero-gc-probability

Allow the 'gc_probability' option to be set to zero.
Merge branch 'hotfix/4608'

Close #4608