VideoTrack::Parse: fix potential memory leak

for consistency this matches the fix in
cb5a947 Fix a potential memory leak in mkvparser.cc

these could both be simplified by transferring the ownership of the
pointer by returning a std::unique_ptr directly or as an output
parameter.

BUG=b/129393729

Change-Id: I196974e475e459acf43cd673e4339e811abb2025
1 file changed