h264: free the tables and uninitialize the context on flush

Prevents referencing empty frames when the first packet after the flush
does not contain a frame.

libavcodec/h264.c

@@ -2768,6 +2768,9 @@ static void flush_dpb(AVCodecContext *avctx)
     h->parse_context.overread_index    = 0;
     h->parse_context.index             = 0;
     h->parse_context.last_index        = 0;
+
+    free_tables(h, 1);
+    h->context_initialized = 0;
 }
 
 int ff_init_poc(H264Context *h, int pic_field_poc[2], int *pic_poc)