matroskademux: header parsing fixes

matroskademux: Initialize track context out parameter to NULL before parsing
Various error return paths don't set it to NULL and callers are only
checking if the pointer is NULL. As it's allocated on the stack this
usually contains random stack memory, and more often than not the memory
of a previously parsed track.

This then causes all kinds of memory corruptions further down the line.

Thanks to Natalie Silvanovich for reporting.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/issues/858

matroskademux: Fix extraction of multichannel WavPack
The old code had a couple of issues that all lead to potential memory
safety bugs.

  - Use a constant for the Wavpack4Header size instead of using sizeof.
    It's written out into the data and not from the struct and who knows
    what special alignment/padding requirements some C compilers have.
  - gst_buffer_set_size() does not realloc the buffer when setting a
    bigger size than allocated, it only allows growing up to the maximum
    allocated size. Instead use a GstAdapter to collect all the blocks
    and take out everything at once in the end.
  - Check that enough data is actually available in the input and
    otherwise handle it an error in all cases instead of silently
    ignoring it.

mentioned in commit tpm/gst-plugins-good@242f3cae

mentioned in commit tpm/gst-plugins-good@6c461e90

added 2 commits

• 6c461e90 - matroskademux: Initialize track context out parameter to NULL before parsing
• 242f3cae - matroskademux: Fix extraction of multichannel WavPack

Compare with previous version

added 1.14 1.16 1.18 Needs backport labels

mentioned in commit tpm/gst-plugins-good@6a03c578

mentioned in commit tpm/gst-plugins-good@7e55e4ad

added Backported into 1.18 label

Merge Request for backport into 1.18 branch: !903 (merged) (target milestone: 1.18.4)

mentioned in merge request !903 (merged)

mentioned in commit tpm/gst-plugins-good@ae545bc6

mentioned in commit tpm/gst-plugins-good@b6c418d9

added Backported into 1.16 label

Merge Request for backport into 1.16 branch: !904 (merged) (target milestone: None)

mentioned in merge request !904 (merged)

mentioned in commit tpm/gst-plugins-good@aad0b6e4

mentioned in commit tpm/gst-plugins-good@fc0c8ad4

added Backported into 1.14 label and removed Needs backport label

Merge Request for backport into 1.14 branch: !905 (merged) (target milestone: None)

mentioned in merge request !905 (merged)

assigned to @tpm and unassigned @gstreamer-merge-bot