Fix the size of workspace buffers in the motion pixels decoder.

Some buffers must be mod 4 in width and/or height.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/motionpixels.c

@@ -52,14 +52,16 @@ typedef struct MotionPixelsContext {
 static av_cold int mp_decode_init(AVCodecContext *avctx)
 {
     MotionPixelsContext *mp = avctx->priv_data;
+    int w4 = (avctx->width  + 3) & ~3;
+    int h4 = (avctx->height + 3) & ~3;
 
     motionpixels_tableinit();
     mp->avctx = avctx;
     dsputil_init(&mp->dsp, avctx);
-    mp->changes_map = av_mallocz(avctx->width * avctx->height);
+    mp->changes_map = av_mallocz(avctx->width * h4);
     mp->offset_bits_len = av_log2(avctx->width * avctx->height) + 1;
     mp->vpt = av_mallocz(avctx->height * sizeof(YuvPixel));
-    mp->hpt = av_mallocz(avctx->height * avctx->width / 16 * sizeof(YuvPixel));
+    mp->hpt = av_mallocz(h4 * w4 / 16 * sizeof(YuvPixel));
     avctx->pix_fmt = PIX_FMT_RGB555;
     avcodec_get_frame_defaults(&mp->frame);
     return 0;