avcodec/gdv: Fix undefined shift

Fixes: runtime error: left shift of 255 by 24 places cannot be represented in type 'int'
Fixes: 2249/clusterfuzz-testcase-minimized-5388542379294720

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/gdv.c

@@ -330,7 +330,7 @@ static int gdv_decode_frame(AVCodecContext *avctx, void *data,
             unsigned r = bytestream2_get_byte(gb);
             unsigned g = bytestream2_get_byte(gb);
             unsigned b = bytestream2_get_byte(gb);
-            gdv->pal[i] = 0xFF << 24 | r << 18 | g << 10 | b << 2;
+            gdv->pal[i] = 0xFFU << 24 | r << 18 | g << 10 | b << 2;
         }
         break;
     case 3: