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.
diff --git a/Python/thread.c b/Python/thread.c
index ec6909c..bac0e69 100644
--- a/Python/thread.c
+++ b/Python/thread.c
@@ -92,7 +92,7 @@
 size_t
 PyThread_get_stacksize(void)
 {
-    return _PyInterpreterState_GET_UNSAFE()->pythread_stacksize;
+    return _PyInterpreterState_GET()->pythread_stacksize;
 }
 
 /* Only platforms defining a THREAD_SET_STACKSIZE() macro