reftable: handle null refnames in reftable_ref_record_equal

Spotted by Coverity.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/record.c

@@ -1154,9 +1154,11 @@ int reftable_ref_record_equal(struct reftable_ref_record *a,
 			      struct reftable_ref_record *b, int hash_size)
 {
 	assert(hash_size > 0);
-	if (!(0 == strcmp(a->refname, b->refname) &&
-	      a->update_index == b->update_index &&
-	      a->value_type == b->value_type))
+	if (!null_streq(a->refname, b->refname))
+		return 0;
+
+	if (a->update_index != b->update_index ||
+	    a->value_type != b->value_type)
 		return 0;
 
 	switch (a->value_type) {