motionpixels: check extradata size

Fixes null ptr derefernce
Fixes Ticket1363
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/motionpixels.c

@@ -55,6 +55,11 @@ static av_cold int mp_decode_init(AVCodecContext *avctx)
     int w4 = (avctx->width  + 3) & ~3;
     int h4 = (avctx->height + 3) & ~3;
 
+    if(avctx->extradata_size < 2){
+        av_log(avctx, AV_LOG_ERROR, "extradata too small\n");
+        return AVERROR_INVALIDDATA;
+    }
+
     motionpixels_tableinit();
     mp->avctx = avctx;
     ff_dsputil_init(&mp->dsp, avctx);