avcodec/snow: Use ff_thread_once() in ff_snow_common_init()

avcodec/snow: Use ff_thread_once() in ff_snow_common_init()

ff_snow_common_init() currently initializes static data every time it is
invoked; given that both the Snow encoder and decoder have the
FF_CODEC_CAP_INIT_THREADSAFE flag set, this can lead to data races (and
therefore undefined behaviour) even though all threads write the same
values. This commit fixes this by using ff_thread_once() for the
initializations.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>