add_commit_patch_id(): avoid allocating memory unnecessarily

It would appear that we allocate (and forget to release) memory if the
patch ID is not even defined.

Reported by the Coverity tool.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

patch-ids.c

@@ -99,11 +99,12 @@ struct patch_id *has_commit_patch_id(struct commit *commit,
 struct patch_id *add_commit_patch_id(struct commit *commit,
 				     struct patch_ids *ids)
 {
-	struct patch_id *key = xcalloc(1, sizeof(*key));
+	struct patch_id *key;
 
 	if (!patch_id_defined(commit))
 		return NULL;
 
+	key = xcalloc(1, sizeof(*key));
 	if (init_patch_id_entry(key, commit, ids)) {
 		free(key);
 		return NULL;