_PyGILState_Init(), PyGILState_Ensure(): Since PyThread_set_key_value()

_PyGILState_Init(), PyGILState_Ensure():  Since PyThread_set_key_value()
can fail, check its return value, and die if it does fail.

_PyGILState_Init():  Assert that the thread doesn't already have an
association for autoTLSkey.  If it does, PyThread_set_key_value() will
ignore the attempt to (re)set the association, which the code clearly
doesn't want.