Fix a compiler warning in _xxsubinterpretermodule.c (#103245)

Fix a compiler warning in _xxsubinterpretermodule.c (#103245)

Fix a (correct) warning about potential uses of uninitialized memory in
_xxsubinterpreter. Unlike newly allocated PyObject structs or global
structs, stack-allocated structs are not initialised, and a few places in
the code expect the _sharedexception struct data to be either NULL or
initialised.