pgsql: Fix low-probability memory leak in regex execution.

Fix low-probability memory leak in regex execution.

After an internal failure in shortest() or longest() while pinning down the
exact location of a match, find() forgot to free the DFA structure before
returning. This is pretty unlikely to occur, since we just successfully
ran the "search" variant of the DFA; but it could happen, and it would
result in a session-lifespan memory leak since this code uses malloc()
directly. Problem seems to have been aboriginal in Spencer's library,
so back-patch all the way.

In passing, correct a thinko in a comment I added awhile back about the
meaning of the "ntree" field.

I happened across these issues while comparing our code to Tcl's version
of the library.

Branch
------
REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/dc4e8c10169e094f4241efa586459a812bdfd393

Modified Files
--------------
src/backend/regex/regcomp.c | 2 +-
src/backend/regex/regexec.c | 6 +++++-
src/include/regex/regguts.h | 2 +-
3 files changed, 7 insertions(+), 3 deletions(-)