regex: re_search etc. should return -2 when memory exhausted

regex: re_search etc. should return -2 when memory exhausted

This bug was uncovered when testing 'grep'.  Without the fix,
re_search and friends return -1 when memory is exhausted, but -1
means no match, and this causes grep to falsely report no-match
instead of memory-exhaustion.  See
<http://sources.redhat.com/bugzilla/show_bug.cgi?id=13762>.
* lib/regexec.c (re_search_stub): Return -2 (not -1) if there is
trouble; this can occur if re_search_internal ran out of memory.