non-null main on subinterp test

Author: neonene

Modules/_testmultiphase.c

@@ -959,15 +959,18 @@ PyInit__test_shared_gil_only(void)
 static int
 datetime_capi_exec(PyObject *m)
 {
-    _pydatetimeapi_main = NULL;
+    int ismain = PyInterpreterState_Get() == PyInterpreterState_Main();
+    if (ismain) {
+        _pydatetimeapi_main = NULL;
+    }
     PyDateTime_IMPORT;
     if (PyDateTimeAPI == NULL) {
         return -1;
     }
     if (PyDateTimeAPI != PyCapsule_Import(PyDateTime_CAPSULE_NAME, 0)) {
         return -1;
     }
-    if (PyInterpreterState_Get() == PyInterpreterState_Main()) {
+    if (ismain) {
         if (PyDateTimeAPI != _pydatetimeapi_main) {
             return -1;
         }