Fix bug #73948: Trailing NULL padding for UNMATCHED_AS_NULL

[nikic]

This is a fix for https://bugs.php.net/bug.php?id=73948, targeting 7.4 for BC concerns.

There's two changes here affecting UNMATCHED_AS_NULL mode:

• Make sure we always have the same number of results. If some of the unmatched subpatterns are at the end, we insert appropriate padding NULLs.
• When using offset capture, instead of creating an array [null, -1], insert just null.

I'm not so sure about the second bit. I think this is how this should work, but possibly we need to stick with the current behavior for BC.

/cc @cmb69

[nikic]

After thinking about this a bit more, I'm dropping the second point. There's value in always having the same format... Now this change only includes padding with null or [null, -1]. I've implemented an optimization in 4fe3d10 that avoids creating a new array for each [null, -1] entry, so doing this shouldn't be much more expensive than using just null.

[nikic]

Merged as 6311581. I think in this form the BC break here should be very limited... let's see if anyone complains :)