Provide a sanity check during PyThreadState_DeleteCurrent() and

Provide a sanity check during PyThreadState_DeleteCurrent() and
PyThreadState_Delete() to avoid an infinite loop when the tstate list
is messed up and has somehow becomes circular and does not contain the
current thread.

I don't know how this happens but it does, *very* rarely.  On more than
one hardware platform.  I have not been able to reproduce it manually.

Attaching to a process where its happening: it has always been in an
infinite loop over a single element tstate list that is not the tstate
we're looking to delete.  It has been in t_bootstrap()'s call to
PyThreadState_DeleteCurrent() as a pthread is exiting.