motionpixels: use meaningful error code

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/motionpixels.c

@@ -252,13 +252,13 @@ static int mp_decode_frame(AVCodecContext *avctx,
     int buf_size = avpkt->size;
     MotionPixelsContext *mp = avctx->priv_data;
     GetBitContext gb;
-    int i, count1, count2, sz;
+    int i, count1, count2, sz, ret;
 
     mp->frame.reference = 3;
     mp->frame.buffer_hints = FF_BUFFER_HINTS_VALID | FF_BUFFER_HINTS_PRESERVE | FF_BUFFER_HINTS_REUSABLE;
-    if (avctx->reget_buffer(avctx, &mp->frame)) {
+    if ((ret = avctx->reget_buffer(avctx, &mp->frame)) < 0) {
         av_log(avctx, AV_LOG_ERROR, "reget_buffer() failed\n");
-        return -1;
+        return ret;
     }
 
     /* le32 bitstream msb first */