avcodec/mpeg12dec: Use 64bit in bit computation

I dont think this can actually overflow but 64bit seems reasonable to use

Fixes: CID1521983 Unintentional integer overflow

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg12dec.c

@@ -2734,7 +2734,7 @@ static int ipu_decode_frame(AVCodecContext *avctx, AVFrame *frame,
     int ret;
 
     // Check for minimal intra MB size (considering mb header, luma & chroma dc VLC, ac EOB VLC)
-    if (avpkt->size*8LL < (avctx->width+15)/16 * ((avctx->height+15)/16) * (2 + 3*4 + 2*2 + 2*6))
+    if (avpkt->size*8LL < (avctx->width+15)/16 * ((avctx->height+15)/16) * (2LL + 3*4 + 2*2 + 2*6))
         return AVERROR_INVALIDDATA;
 
     ret = ff_get_buffer(avctx, frame, 0);