server: don't assert on invalid client message

Some users have been reaching this error:
snd_receive: ASSERT n failed

A misbehaving client could easily hit that condition by sending too big
messages. Instead of assert(), replace with a warning. When a message
too big to fit is received, it will simply disconnect the channel.

https://bugzilla.redhat.com/show_bug.cgi?id=962187

server/snd_worker.c

@@ -421,7 +421,7 @@ static void snd_receive(void* data)
     for (;;) {
         ssize_t n;
         n = channel->receive_data.end - channel->receive_data.now;
-        spice_assert(n);
+        spice_warn_if(n <= 0);
         n = reds_stream_read(channel->stream, channel->receive_data.now, n);
         if (n <= 0) {
             if (n == 0) {