avcodec/xsubdec: fix overflow in alpha handling

Fixes: left shift of 255 by 24 places cannot be represented in type 'int'
Fixes: 18368/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XSUB_fuzzer-5702665442426880

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

libavcodec/xsubdec.c

@@ -130,7 +130,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
             ((uint32_t *)sub->rects[0]->data[1])[i] |= 0xff000000;
     } else {
         for (i = 0; i < sub->rects[0]->nb_colors; i++)
-            ((uint32_t *)sub->rects[0]->data[1])[i] |= *buf++ << 24;
+            ((uint32_t *)sub->rects[0]->data[1])[i] |= (unsigned)*buf++ << 24;
     }
 
 #if FF_API_AVPICTURE