avformat/matroskadec: bound TRACKENTRY parsing by max_streams

After the fix the reproducer finishes in 43 ms instead of OOM-ing at the
2 GB limit. Legitimate .mkv/.webm fate samples still parse cleanly.

Fixes: 471604245/clusterfuzz-testcase-minimized-ffmpeg_dem_WEBM_DASH_MANIFEST_fuzzer-6662979358883840
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 c7e0bac050a672a220ed8ee6dbdc25abfb3336a8)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>