jsonrpc: fix cppcheck warning for potential null pointer deref

cppcheck was failing with a warning for a null pointer deref at this
line, this makes it stop complaining (and lets make check-source
succeed)

lightningd/jsonrpc.c

@@ -319,6 +319,7 @@ static void connection_complete_error(struct json_connection *jcon,
 
 	assert(id != NULL);
 
+	assert(cmd);
 	if (cmd->ok)
 		*(cmd->ok) = false;