monitor: Fix crash

NLMSG_OK and NLMSG_NEXT expect to operate on nlmsg_len which is an int
(signed type).  The current code uses an unsigned type which means that
it cannot detect underflows.  Such underflows can happen when NLMSG_NEXT
tries to advance nlmsg_len by a number of bytes (due to alignment) which
are greater than the current nlmsg_len itself.  This causes iwmon to
crash on certain messages.

Reported-By: Daniel Wagner <wagi@monom.org>

monitor/nlmon.c

@@ -6876,7 +6876,7 @@ static bool nlmon_receive(struct l_io *io, void *user_data)
 	unsigned char buf[8192];
 	unsigned char control[32];
 	ssize_t bytes_read;
-	size_t nlmsg_len;
+	int nlmsg_len;
 	int fd;
 
 	fd = l_io_get_fd(io);