avcodec/cbs: Remove redundant checks for CodedBitstreamContext.codec

avcodec/cbs: Remove redundant checks for CodedBitstreamContext.codec

Setting this field happens immediately after the allocation in
ff_cbs_init(), so the whole CBS code may presume that any
CodedBitstreamContext has this set. Lots of code already presumed this,
yet ff_cbs_close() did it inconsistently: It checked before checking
whether the CodedBitstreamType has a close function; yet it simply
unconditionally read ctx->codec->priv_class. Coverity complained about
this in issue #1473564, which this commit fixes.

Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>