pgsql: Fix regexp_matches() handling of zero-length matches.

Fix regexp_matches() handling of zero-length matches.

We'd find the same match twice if it was of zero length and not immediately
adjacent to the previous match. replace_text_regexp() got similar cases
right, so adjust this search logic to match that. Note that even though
the regexp_split_to_xxx() functions share this code, they did not display
equivalent misbehavior, because the second match would be considered
degenerate and ignored.

Jeevan Chalke, with some cosmetic changes by me.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/8505ebf31eb50c20afeaca0e0dfa2fbb1cec001f

Modified Files
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src/backend/utils/adt/regexp.c | 13 ++++----
src/backend/utils/adt/varlena.c | 5 ++-
src/test/regress/expected/strings.out | 58 +++++++++++++++++++++++++++++++++
src/test/regress/sql/strings.sql | 7 ++++
4 files changed, 75 insertions(+), 8 deletions(-)