The usage of a static variable presents a potential for data races and
means that this function can't be used in init functions of codecs with
FF_CODEC_CAP_INIT_THREADSAFE (unless of course one presumes that
everything is alright in which case the error is not triggered; but then
the whole function is pointless...). This makes the Snow decoder
init-threadsafe as it already claims.

Notice that this function has been removed in 2014 by Libav in commit
9103185b, because only some codepaths
are checked this way and because it only affects legacy compilers. The
latter is of course even more true today.
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>