xldec: Check that width is a multiple of 4

Fixes out of array reads

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

libavcodec/xl.c

@@ -52,6 +52,11 @@ static int decode_frame(AVCodecContext *avctx,
     uint32_t val;
     int y0, y1, y2, y3 = 0, c0 = 0, c1 = 0;
 
+    if (avctx->width & 3) {
+        av_log(avctx, AV_LOG_ERROR, "width is not a multiple of 4\n");
+        return AVERROR_INVALIDDATA;
+    }
+
     if(p->data[0])
         avctx->release_buffer(avctx, p);