pictor: use the correct logging context

Broken in 6d97484d72

libavcodec/pictordec.c

@@ -123,7 +123,7 @@ static int decode_frame(AVCodecContext *avctx,
     s->nb_planes   = (tmp >> 4) + 1;
     bpp            = bits_per_plane * s->nb_planes;
     if (bits_per_plane > 8 || bpp < 1 || bpp > 32) {
-        avpriv_request_sample(s, "Unsupported bit depth");
+        avpriv_request_sample(avctx, "Unsupported bit depth");
         return AVERROR_PATCHWELCOME;
     }