avformat/icodec: Check nb_pal

Fixes: signed integer overflow: 538976288 * 4 cannot be represented in type 'int'
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_ICO_fuzzer-6690068904935424

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/icodec.c

@@ -196,6 +196,9 @@ static int read_packet(AVFormatContext *s, AVPacket *pkt)
             AV_WL32(buf + 32, image->nb_pal);
         }
 
+        if (image->nb_pal > INT_MAX / 4 - 14 - 40)
+            return AVERROR_INVALIDDATA;
+
         AV_WL32(buf - 4, 14 + 40 + image->nb_pal * 4);
         AV_WL32(buf + 8, AV_RL32(buf + 8) / 2);
     }