bpo-40268: Rename _PyInterpreterState_GET_UNSAFE() (GH-19509)

Rename _PyInterpreterState_GET_UNSAFE() to _PyInterpreterState_GET()
for consistency with _PyThreadState_GET() and to have a shorter name
(help to fit into 80 columns).

Add also "assert(tstate != NULL);" to the function.

Python/thread_pthread.h

@@ -735,7 +735,7 @@ _pythread_pthread_set_stacksize(size_t size)
 
     /* set to default */
     if (size == 0) {
-        _PyInterpreterState_GET_UNSAFE()->pythread_stacksize = 0;
+        _PyInterpreterState_GET()->pythread_stacksize = 0;
         return 0;
     }
 
@@ -752,7 +752,7 @@ _pythread_pthread_set_stacksize(size_t size)
             rc = pthread_attr_setstacksize(&attrs, size);
             pthread_attr_destroy(&attrs);
             if (rc == 0) {
-                _PyInterpreterState_GET_UNSAFE()->pythread_stacksize = size;
+                _PyInterpreterState_GET()->pythread_stacksize = size;
                 return 0;
             }
         }