read_ref() can (and in test t5800, actually *does*) return NULL.
Don't pass the NULL along to read_ref(). Coincidentally, this mistake
didn't make resolve_ref() blow up, but upcoming changes to
resolve_ref() will make it less forgiving.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>