mkvparser,VideoTrack::Parse: fix leak on error
mkvparser,VideoTrack::Parse: fix leak on error

of colour_space; use a unique_ptr to capture the return from
UnserializeString. this is similar to other fixes in this function.

Bug: b/152133942
Change-Id: I6ff4c8ed278b782717a92bb21977740a61205931

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm