roqvideodec: check dimensions validity

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

libavcodec/roqvideodec.c

@@ -170,6 +170,12 @@ static av_cold int roq_decode_init(AVCodecContext *avctx)
     RoqContext *s = avctx->priv_data;
 
     s->avctx = avctx;
+
+    if (avctx->width%16 || avctx->height%16) {
+         av_log_ask_for_sample(avctx, "dimensions not being a multiple of 16 are unsupported\n");
+         return AVERROR_PATCHWELCOME;
+    }
+
     s->width = avctx->width;
     s->height = avctx->height;
     avcodec_get_frame_defaults(&s->frames[0]);