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)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/h264.c

@@ -686,6 +686,7 @@ 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->rbsp_buffer[0]      = NULL;
     h->rbsp_buffer[1]      = NULL;
     h->rbsp_buffer_size[0] = 0;