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/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 938df24..28ec8f1 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -436,7 +436,7 @@
         return 0;
     }
 
-    PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
+    PyInterpreterState *interp = _PyInterpreterState_GET();
 #ifndef Py_DEBUG
     /* In release mode, only check in development mode (-X dev) */
     if (!_PyInterpreterState_GetConfig(interp)->dev_mode) {
@@ -3615,7 +3615,7 @@
 PyObject *
 PyUnicode_EncodeFSDefault(PyObject *unicode)
 {
-    PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
+    PyInterpreterState *interp = _PyInterpreterState_GET();
     if (interp->fs_codec.utf8) {
         return unicode_encode_utf8(unicode,
                                    interp->fs_codec.error_handler,
@@ -3851,7 +3851,7 @@
 PyObject*
 PyUnicode_DecodeFSDefaultAndSize(const char *s, Py_ssize_t size)
 {
-    PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
+    PyInterpreterState *interp = _PyInterpreterState_GET();
     if (interp->fs_codec.utf8) {
         return unicode_decode_utf8(s, size,
                                    interp->fs_codec.error_handler,
@@ -16009,7 +16009,7 @@
 int
 _PyUnicode_EnableLegacyWindowsFSEncoding(void)
 {
-    PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
+    PyInterpreterState *interp = _PyInterpreterState_GET();
     PyConfig *config = (PyConfig *)_PyInterpreterState_GetConfig(interp);
 
     /* Set the filesystem encoding to mbcs/replace (PEP 529) */