matroskadec: Fix read-after-free in matroska_read_seek()

In matroska_read_seek(), |tracks| is assigned at the begining of the
function. However, functions like matroska_parse_cues() could reallocate
the tracks and invalidate |tracks|.

This assigns |tracks| only before using it, so that it will not get
invalidated elsewhere.

Bug-Id: chromium/427266
