Fix regexp substring matching (substring(string from pattern)) for the corner

Fix regexp substring matching (substring(string from pattern)) for the corner
case where there is a match to the pattern overall but the user has specified
a parenthesized subexpression and that subexpression hasn't got a match.
An example is substring('foo' from 'foo(bar)?').  This should return NULL,
since (bar) isn't matched, but it was mistakenly returning the whole-pattern
match instead (ie, 'foo').  Per bug #4044 from Rui Martins.

This has been broken since the beginning; patch in all supported versions.
The old behavior was sufficiently inconsistent that it's impossible to believe
anyone is depending on it.