avformat/bfi: check nframes

Fixes: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
Fixes: 29102/clusterfuzz-testcase-minimized-ffmpeg_dem_BFI_fuzzer-6737028768202752

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

libavformat/bfi.c

@@ -73,6 +73,8 @@ static int bfi_read_header(AVFormatContext * s)
         return AVERROR_INVALIDDATA;
 
     bfi->nframes           = avio_rl32(pb);
+    if (bfi->nframes < 0)
+        return AVERROR_INVALIDDATA;
     avio_rl32(pb);
     avio_rl32(pb);
     avio_rl32(pb);