roqvideodec: check dimensions validity

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 3ae61045)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/roqvideodec.c

@@ -158,6 +158,12 @@ 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(avctx, AV_LOG_ERROR, "dimensions not being a multiple of 16 are unsupported\n");
+         return AVERROR_PATCHWELCOME;
+    }
+
     s->width = avctx->width;
     s->height = avctx->height;
     s->last_frame    = &s->frames[0];