clone.c: don't leak memory in cmd_clone

Free the refspec.
Found by scan.coverity.com (Id: 1127806)
Signed-off-by: NStefan Beller <stefanbeller@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -1000,5 +1000,7 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	strbuf_release(&key);
 	strbuf_release(&value);
 	junk_mode = JUNK_LEAVE_ALL;
+
+	free(refspec);
 	return err;
 }