xxan: Disallow odd width

Decoded data is always written in pairs within this decoder.
This fixes writes out of bounds.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/xxan.c

@@ -50,6 +50,10 @@ static av_cold int xan_decode_init(AVCodecContext *avctx)
         av_log(avctx, AV_LOG_ERROR, "Invalid frame height: %d.\n", avctx->height);
         return AVERROR(EINVAL);
     }
+    if (avctx->width & 1) {
+        av_log(avctx, AV_LOG_ERROR, "Invalid frame width: %d.\n", avctx->width);
+        return AVERROR(EINVAL);
+    }
 
     s->buffer_size = avctx->width * avctx->height;
     s->y_buffer = av_malloc(s->buffer_size);