- Wrap MkvWriter in std::unique_ptr to allow use of the
MkvWriter(FILE*) ctor (because MuxerTest owns the FILE*).
- Test temporary FILE* in Init(); don't assume it's valid.
- Close output FILE* before attempting to read it in
CuesBeforeClusters().
Change-Id: I4848adec7a24c8987fdd0924e7474b264837e8a9