Check av_dup_packet() return code

Fixes: CID1338320
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/frame_thread_encoder.c

@@ -89,7 +89,9 @@ static void * attribute_align_arg worker(void *v){
         pthread_mutex_unlock(&c->buffer_mutex);
         av_frame_free(&frame);
         if(got_packet) {
-            av_dup_packet(pkt);
+            int ret2 = av_dup_packet(pkt);
+            if (ret >= 0 && ret2 < 0)
+                ret = ret2;
         } else {
             pkt->data = NULL;
             pkt->size = 0;