pthreads: fix segfault due to the thread beimg killed before it has been allocated

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pthread.c

@@ -658,7 +658,8 @@ static void frame_thread_free(AVCodecContext *avctx, int thread_count)
         pthread_cond_signal(&p->input_cond);
         pthread_mutex_unlock(&p->mutex);
 
-        pthread_join(p->thread, NULL);
+        if(p->thread)
+            pthread_join(p->thread, NULL);
 
         if (codec->close)
             codec->close(p->avctx);