avcodec/vp9: fix "initialize" typo.

Originally spotted and fixed by one of these developers:
    Anton Khirnov <anton@khirnov.net>
    Diego Biurrun <diego@biurrun.de>
    Luca Barbato <lu_zero@gentoo.org>
    Martin Storsjö <martin@martin.st>

See 97962b2 / 72ca830
lavf/matroska*: DiscardPadding is a signed integer

according to the Matriska Specification
 http://matroska.org/technical/specs/index.html
DiscardPadding is a signed integer.

Tested-by: Jan Gerber <j@v2v.cc>
Tested-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>