tiffenc: initialize forgotten avctx.

libavcodec/tiffenc.c

@@ -220,6 +220,7 @@ static int encode_frame(AVCodecContext * avctx, unsigned char *buf,
     uint8_t *yuv_line = NULL;
     int shift_h, shift_v;
 
+    s->avctx = avctx;
     s->buf_start = buf;
     s->buf = &ptr;
     s->buf_size = buf_size;