avformat/icodec: Check size

Fixes: signed integer overflow: 14 + 2147483647 cannot be represented in type 'int'
Fixes: 471688026/clusterfuzz-testcase-minimized-ffmpeg_dem_ICO_fuzzer-5616495813263360

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 237d03717f)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/icodec.c

@@ -114,7 +114,7 @@ static int read_header(AVFormatContext *s)
         avio_skip(pb, 5);
 
         ico->images[i].size   = avio_rl32(pb);
-        if (ico->images[i].size <= 0) {
+        if (ico->images[i].size <= 0 || ico->images[i].size > INT_MAX - 14) {
             av_log(s, AV_LOG_ERROR, "Invalid image size %d\n", ico->images[i].size);
             goto fail;
         }