When fpcurthread is not the current thread it may be non-NULL. In this
case another thread has had the VFP unit enabled and will have its state
in the VFP registers along with it stored in memory. As such we don't need
to store the state, but do need to zero the fpcurthread pointer to stop
the VFP driver from using the enable fast path.
Reported by: emaste
Sponsored by: DARPA, AFRL