avformat/dxa: check bpc

Fixes: integer overflow: -2147483648 - 1 cannot be represented in type 'int'
Fixes: 373971762/clusterfuzz-testcase-minimized-ffmpeg_dem_DXA_fuzzer-4880491112103936

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 7e020f2141)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/dxa.c

@@ -120,6 +120,8 @@ static int dxa_read_header(AVFormatContext *s)
             avio_skip(pb, fsize);
         }
         c->bpc = (fsize + (int64_t)c->frames - 1) / c->frames;
+        if (c->bpc < 0)
+            return AVERROR_INVALIDDATA;
         if(ast->codecpar->block_align) {
             if (c->bpc > INT_MAX - ast->codecpar->block_align + 1)
                 return AVERROR_INVALIDDATA;