utvideoenc: fix theoretical integer overflow

The image sizes needed for this overflow to happen are currently not
supported in lavc, thus this should have no effect execpt making the
code more robust in light of future changes.

Fixes CID732245
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utvideoenc.c

@@ -417,7 +417,7 @@ static int encode_plane(AVCodecContext *avctx, uint8_t *src,
         /* If non-zero count is found, see if it matches width * height */
         if (counts[symbol]) {
             /* Special case if only one symbol was used */
-            if (counts[symbol] == width * height) {
+            if (counts[symbol] == width * (int64_t)height) {
                 /*
                  * Write a zero for the single symbol
                  * used in the plane, else 0xFF.