h264: initialize H264Context.avctx in init_thread_copy

This prevents using a wrong (first thread's) AVCodecContext if decoding
a frame in the first pass over all threads fails.
(cherry picked from commit a06b0b1295)

Conflicts:

	libavcodec/h264.c

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

libavcodec/h264.c

@@ -1588,6 +1588,7 @@ static int decode_init_thread_copy(AVCodecContext *avctx)
     memset(h->sps_buffers, 0, sizeof(h->sps_buffers));
     memset(h->pps_buffers, 0, sizeof(h->pps_buffers));
 
+    h->avctx = avctx;
     h->context_initialized = 0;
 
     return 0;