Fix memory (re)allocation in matroskadec.c, related to MSVR-11-0080.

Whitespace of the patch cleaned up by Aurel
Some of the issues have been reported by Steve Manzuik / Microsoft Vulnerability Research (MSVR)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

(cherry picked from commit 956c901c68eff78288f40e3c8f41ee2fa081d4a8)

Further suggestions from Kostya <kostya.shishkov@gmail.com> have been
implemented by Reinhard Tartler <siretart@tauware.de>

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