MINOR: sock: add a check against cross worker<->master socket activities

Given that the previous issues caused spurious worker socket wakeups in
the master for inherited FDs that couldn't be closed, let's add a strict
test in the I/O callback to make sure that an accept() event is always
caught by the appropriate type of process (master for master listeners,
worker for worker listeners).

src/sock.c

@@ -621,6 +621,7 @@ void sock_accept_iocb(int fd)
 	if (!l)
 		return;
 
+	BUG_ON(!!master != !!(l->rx.flags & RX_F_MWORKER));
 	listener_accept(l);
 }