Fix memory leak during regular expression execution.

For a regex containing backrefs, pg_regexec() might fail to free all the
sub-DFAs that were created during execution, resulting in a permanent
(session lifespan) memory leak.  Problem was introduced by me in commit
587359479acbbdc95c8e37da40707e37097423f5.  Per report from Sandro Santilli;
diagnosis by Greg Stark.