pthread: Fix crash due to fctx->delaying not being cleared.

Reproducible with test case and ffplay -threads 2. Stack trace:
http://pastebin.com/PexZ4Uc0

Test case:
http://commondatastorage.googleapis.com/dalecurtis-shared/crash.ogmSigned-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/pthread.c

@@ -603,11 +603,12 @@ int ff_thread_decode_frame(AVCodecContext *avctx,
      * If we're still receiving the initial packets, don't return a frame.
      */
 
-    if (fctx->delaying && avpkt->size) {
+    if (fctx->delaying) {
         if (fctx->next_decoding >= (avctx->thread_count-1)) fctx->delaying = 0;
 
         *got_picture_ptr=0;
-        return avpkt->size;
+        if (avpkt->size)
+            return avpkt->size;
     }
 
     /*