slirp: check for ioctlsocket error and 0-length udp payload.

Sometimes sorecvfrom() is called from slirp.c because revents == G_IO_IN,
but there is 0 bytes available and recvfrom could be blocking indefinitely.
This is likely due to 0-length udp payload. This also adds an error
checking for ioctlsocket.
Signed-off-by: NVic Lee <llyzs.vic@gmail.com>
Message-Id: <20190301064809.3074-1-llyzs.vic@gmail.com>
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>

slirp/socket.c

@@ -529,6 +529,15 @@ sorecvfrom(struct socket *so)
           int n;
 #endif
 
+	  if (ioctlsocket(so->s, FIONREAD, &n) != 0) {
+	      DEBUG_MISC(" ioctlsocket errno = %d-%s\n",
+			 errno,strerror(errno));
+	      return;
+	  }
+	  if (n == 0) {
+	      return;
+	  }
+
 	  m = m_get(so->slirp);
 	  if (!m) {
 	      return;
@@ -552,7 +561,6 @@ sorecvfrom(struct socket *so)
 	   */
 	  len = M_FREEROOM(m);
 	  /* if (so->so_fport != htons(53)) { */
-	  ioctlsocket(so->s, FIONREAD, &n);
 
 	  if (n > len) {
 	    n = (m->m_data - m->m_dat) + m->m_len + n + 1;