pgsql: Fix regex match failures for backrefs combined with non-greedy q

Fix regex match failures for backrefs combined with non-greedy quantifiers.

An ancient logic error in cfindloop() could cause the regex engine to fail
to find matches that begin later than the start of the string. This
function is only used when the regex pattern contains a back reference,
and so far as we can tell the error is only reachable if the pattern is
non-greedy (i.e. its first quantifier uses the ? modifier). Furthermore,
the actual match must begin after some potential match that satisfies the
DFA but then fails the back-reference's match test.

Reported and fixed by Jeevan Chalke, with cosmetic adjustments by me.

Branch
------
REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f8463fba2434d083bb699d1cc8a7d7159d614a7b

Modified Files
--------------
src/backend/regex/regexec.c | 20 +++++++++++---------
src/test/regress/expected/regex.out | 15 +++++++++++++++
src/test/regress/sql/regex.sql | 5 +++++
3 files changed, 31 insertions(+), 9 deletions(-)