sv.c:sv_pos_u2b: Don’t cache anything on magical SVs

sv.c:sv_pos_u2b: Don’t cache anything on magical SVs

This should not change the behaviour.  It is just unnecessary to cache
positions on a magical string, as we then just have to invalidate the
cache in mg_get.  It will be more efficient just not to create the
cache to begin with.

This does not allow us to remove the cache invalidation in mg_get,
because the cache can sometimes be created while magic is being called
and SvMAGICAL is off.  It just avoids extra work in those cases that
are detectable.