Commits 2122f8b9 ("rev-parse: Add support for the ^! and ^@ syntax",
2008-07-26) and 3dd4e732 ("Teach rev-parse the ... syntax.", 2006-07-04)
taught rev-parse new syntax, and used lookup_commit_reference() as part of
their logic.  Neither usage checked the returned commit to see if it was
non-NULL before using it.  Check for NULL and ensure an appropriate error
is reported to the user.

Reported by Florian Weimer and Todd Zullinger.
Helped-by: NJeff King <peff@peff.net>
Signed-off-by: NElijah Newren <newren@gmail.com>
Reviewed-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>