commit: use xstrdup() in get_merge_parent()

Handle allocation errors for the name member just like we already do
for the struct merge_remote_desc itself.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -1589,7 +1589,7 @@ struct commit *get_merge_parent(const char *name)
 		struct merge_remote_desc *desc;
 		desc = xmalloc(sizeof(*desc));
 		desc->obj = obj;
-		desc->name = strdup(name);
+		desc->name = xstrdup(name);
 		commit->util = desc;
 	}
 	return commit;