avcodec/smacker: Use better nb_codes estimate when initializing VLC

avcodec/smacker: Use better nb_codes estimate when initializing VLC

Using the real number of read codes allows to leave a loop in
ff_init_vlc_sparse earlier; notice that all codes not explicitly
set by reading data have been set to zero earlier (i.e. they are
zero-length codes) and such codes are ignored by ff_init_vlc_sparse.

This improves performance: When compiled with GCC 9, the time spent on
one call to smka_decode_frame() for the sample from ticket #2425
decreased from 2195367 decicycles to 2068389 decicycles. For Clang 9,
it improved from 1602075 to 1534188 decicycles. These tests have been
performed 20 times and each times the input file has been looped
32 times to get a sufficient number of frames.

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