avcodec/cdgraphics: avoid signed overflow in alpha

Fixes: left shift of 255 by 24 places cannot be represented in type 'int'
Fixes: 42766/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CDGRAPHICS_fuzzer-5142826105569280

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cdgraphics.c

@@ -122,7 +122,7 @@ static void cdg_load_palette(CDGraphicsContext *cc, uint8_t *data, int low)
         r = ((color >> 8) & 0x000F) * 17;
         g = ((color >> 4) & 0x000F) * 17;
         b = ((color     ) & 0x000F) * 17;
-        palette[i + array_offset] = cc->alpha[i + array_offset] << 24 | r << 16 | g << 8 | b;
+        palette[i + array_offset] = (uint32_t)cc->alpha[i + array_offset] << 24 | r << 16 | g << 8 | b;
     }
     cc->frame->palette_has_changed = 1;
 }