UDP: dont use thread_t as truth value.

Unbreak compile on mingw.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/udp.c

@@ -534,7 +534,7 @@ static int udp_read(URLContext *h, uint8_t *buf, int size)
     fd_set rfds;
     struct timeval tv;
 
-    if (s->circular_buffer_thread) {
+    if (s->fifo) {
 
         do {
             avail = av_fifo_size(s->fifo);