avformat/matroskadec: Check desc_bytes so bits fit in 64bit

Likely a tighter check can be done

Fixes: signed integer overflow: 3305606804154370442 * 8 cannot be represented in type 'long'
Fixes: 70449/clusterfuzz-testcase-minimized-ffmpeg_dem_WEBM_DASH_MANIFEST_fuzzer-4771166007918592

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 c4122406f6)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskadec.c

@@ -4633,7 +4633,7 @@ static int64_t webm_dash_manifest_compute_bandwidth(AVFormatContext *s, int64_t
                 int64_t desc_bytes = desc_end.end_offset - desc_beg.start_offset;
                 int64_t desc_ns = desc_end.end_time_ns - desc_beg.start_time_ns;
                 double desc_sec, calc_bits_per_second, percent, mod_bits_per_second;
-                if (desc_bytes <= 0)
+                if (desc_bytes <= 0 || desc_bytes > INT64_MAX/8)
                     return -1;
 
                 desc_sec = desc_ns / nano_seconds_per_second;