mesa: check if TNL state is null in _tnl_free_vertices() to avoid potential segfault

mesa: check if TNL state is null in _tnl_free_vertices() to avoid potential segfault

_tnl_free_vertices() is called from several places during context tear-down.
Depending on the order in which the swrast, swrast_setup and tnl context is
destroyed we could hit a null pointer here.  This doesn't seem to be an
actual issue with any Mesa drivers, but let's be safe.