grep: handle deref_tag() returning NULL

grep: handle deref_tag() returning NULL

deref_tag() can return NULL.  Exit gracefully in that case instead
of blindly dereferencing the return value.

.name shouldn't ever be NULL, but grep_object() handles that case
explicitly, so let's be defensive here as well and show the broken
object's ID if it happens to lack a name after all.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>