avcodec/smacker: Don't zero-initialize unnecessarily

avcodec/smacker: Don't zero-initialize unnecessarily

With the possible exception of the "last" values when decoding video,
only the part that is actually initialized with values derived from the
bitstream is used afterwards, so it is unnecessary to zero everything at
the beginning. This is also no problem for the "last" values at all,
because they are reset for every frame anyway.

While at it, use sizeof(variable) instead of sizeof(type).

Performance increased slightly: For GCC, from 2068389 decicycles per call
to smka_decode_frame() when decoding the sample from ticket #2425 to 2053758
decicycles; for Clang, from 1534188 to 1523153 decicycles.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>