pgsql: Fix regexp substring matching (substring(string from pattern))

Log Message:
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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.

Tags:
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REL8_0_STABLE

Modified Files:
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    pgsql/src/backend/utils/adt:
        regexp.c (r1.56.4.2 -> r1.56.4.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/regexp.c?r1=1.56.4.2&r2=1.56.4.3)