substr_compare(): Allow zero length comparison

[datibbaw]

Analogous to how strncmp() handles a zero length comparison, substr_compare() with a zero length should return 0 as well.

Example:

substr_compare("abc", "a", 0, 0); // 0
substr_compare("abc", "a", 0, -1); // false (with warning)

This patch also cleans up logic prior to bug #33605

[realityking]

I believe this has been merged with 2e459be

[php-pulls]

Comment on behalf of datibbaw at php.net:

Merged in 5.5 onwards.

[datibbaw]

@realityking Yeah, I've merged it from 5.5 branch onwards, but had to cherry pick the commits :)