avformat/mov: Change the type of the r/g/b variables

Fixes integer overflow
Fixes: unknown_unknown_31b_795_cov_1818643045_raybauduc.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -1710,7 +1710,7 @@ static void mov_parse_stsd_video(MOVContext *c, AVIOContext *pb,
     if ((color_depth == 2) || (color_depth == 4) || (color_depth == 8)) {
         /* for palette traversal */
         unsigned int color_start, color_count, color_end;
-        unsigned char a, r, g, b;
+        unsigned int a, r, g, b;
 
         if (color_greyscale) {
             int color_index, color_dec;