matroskadec: Fix a bug where a pointer was cached to an array that might later...

matroskadec: Fix a bug where a pointer was cached to an array that might later move due to a realloc()

Fixes bug #190
Chromium bug #100492
related to CVE-2011-3893

Signed-off-by: Reinhard Tartler <siretart@tauware.de>