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.

Modules/_xxsubinterpretersmodule.c

@@ -481,7 +481,7 @@ _run_script_in_interpreter(PyObject *mod, PyInterpreterState *interp,
     }
 
     // Run the script.
-    _sharedexception exc;
+    _sharedexception exc = {NULL, NULL};
     int result = _run_script(interp, codestr, shared, &exc);
 
     // Switch back.