Thanks for the fix! I'm wondering whether this same segfault could appear in other places. Do you understand why tstate becomes NULL in this case? (I don't.)

@JelleZijlstra
I am not the expert of this area, but AFAIK if the GIL is released, _PyThreadState_GET can return NULL.
I 've checked some codebase of CPython.
And I 've noticed that PyGILState_GetThisThreadState is used for fallback logic.
PyGILState_GetThisThreadState returns the tstate even if the GIL is released.
But unfortunately PyGILState_GetThisThreadState does not support subinterpreters yet.

@vstinner
PyGILState_GetThisThreadState will support the subinterpereter when every subinterpreters have their own GIL? (I mean near future)

@vstinner will submit the better solution.