Heap-based buffer overflow in GStreamer

Summary

DOS / potential heap overwrite in mkv demuxing using zlib decompression. Integer overflow in matroskademux element in gst_matroska_decompress_data function which causes a segfault, or could cause a heap overwrite, depending on libc and OS. Depending on the libc used, and the underlying OS capabilities, it could be just a segfault or a heap overwrite. If the libc uses mmap for large chunks, and the OS supports mmap, then it is just a segfault (because the realloc before the integer overflow will use mremap to reduce the size of the chunk, and it will start to write to unmapped memory). However, if using a libc implementation that does not use mmap, or if the OS does not support mmap while using libc, then this could result in a heap overwrite.