avcodec/g2meet, mjpegdec: Factor out common VLC initialization code

avcodec/g2meet, mjpegdec: Factor out common VLC initialization code

While just at it, remove the nb_codes parameter: It is redundant
(the number of codes is implicitly contained in the array containing how
many entries of a specific size there are) and for this reason it might
even be wrong, so it is better to check what is actually used instead.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>