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