pthread: Do not use a half updated context as master for deallocation.

Fixes assertion failure

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pthread.c

@@ -754,7 +754,11 @@ static void frame_thread_free(AVCodecContext *avctx, int thread_count)
     park_frame_worker_threads(fctx, thread_count);
 
     if (fctx->prev_thread && fctx->prev_thread != fctx->threads)
-        update_context_from_thread(fctx->threads->avctx, fctx->prev_thread->avctx, 0);
+        if (update_context_from_thread(fctx->threads->avctx, fctx->prev_thread->avctx, 0) < 0) {
+            av_log(avctx, AV_LOG_ERROR, "Final thread update failed\n");
+            fctx->prev_thread->avctx->internal->is_copy = fctx->threads->avctx->internal->is_copy;
+            fctx->threads->avctx->internal->is_copy = 1;
+        }
 
     fctx->die = 1;