git-show-ref --verify: Fail if called without a reference

builtin-show-ref.c (cmd_show_ref): Fail if called with --verify option but
without a reference.
Signed-off-by: NDmitry V. Levin <ldv@altlinux.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin-show-ref.c

@@ -221,9 +221,11 @@ int cmd_show_ref(int argc, const char **argv, const char *prefix)
 	}
 
 	if (verify) {
-		unsigned char sha1[20];
-
+		if (!pattern)
+			die("--verify requires a reference");
 		while (*pattern) {
+			unsigned char sha1[20];
+
 			if (!strncmp(*pattern, "refs/", 5) &&
 			    resolve_ref(*pattern, sha1, 1, NULL)) {
 				if (!quiet)