Fix some rather complex regex issues found on OpenBSD as part of some
ongoing work to fix a sed(1) bug.
Curiously the OpenBSD tests don't trigger segfaults on FreeBSD but the
bugs were confirmed by running a port of FreeBSD's regex under OpenBSD's
malloc. Huge thanks to Ingo for confirming the behavior.
Taken from: Ingo Schwarze (through openbsd-tech 2016-05-15)
MFC after: 1 week
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