flacdec: Fix used AVCodecContext

Fixes out of array writes with multiple threads

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

libavcodec/flacdec.c

@@ -498,6 +498,7 @@ static int flac_decode_frame(AVCodecContext *avctx, void *data,
     int ret;
 
     *got_frame_ptr = 0;
+    s->avctx = avctx;
 
     if (s->max_framesize == 0) {
         s->max_framesize =