avcodec/vp9: add a never triggerable assert.

vp8_rac_get_tree() is called with a tree of size 3, so the returned
value can not be outside [0;3]. All of the [0;3] cases are handled in
the switch, so the assert should not be triggerable by any means. A
similar change was introduced in 97962b2 / 72ca830, with the
av_log()+AVERROR_INVALIDDATA form, suggesting it could be triggerable
somehow. This assert might help static analyzer, or simply the reader.
avcodec/vp9: remove some reset-to-zero from vp9_decode_free().

Those should not be necessary.

Original change 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
avcodec/vp9: assert on sane size in update_size().

w and h are both read as uint16 + 1 so this can not happen. A similar
change was introduced in 97962b2 / 72ca830, with the
av_log()+AVERROR_INVALIDDATA form, suggesting it could be triggerable
somehow.

Change suggested by Ronald S. Bultje.