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/Parser/listnode.c b/Parser/listnode.c
index f53b265..3bcc03e 100644
--- a/Parser/listnode.c
+++ b/Parser/listnode.c
@@ -3,7 +3,7 @@
 
 #include "Python.h"
 #include "pycore_interp.h"   // PyInterpreterState.parser
-#include "pycore_pystate.h"  // _PyInterpreterState_GET_UNSAFE
+#include "pycore_pystate.h"  // _PyInterpreterState_GET
 #include "token.h"
 #include "node.h"
 
@@ -20,7 +20,7 @@
 static void
 listnode(FILE *fp, node *n)
 {
-    PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
+    PyInterpreterState *interp = _PyInterpreterState_GET();
 
     interp->parser.listnode.level = 0;
     interp->parser.listnode.atbol = 1;
@@ -40,7 +40,7 @@
             list1node(fp, CHILD(n, i));
     }
     else if (ISTERMINAL(TYPE(n))) {
-        interp = _PyInterpreterState_GET_UNSAFE();
+        interp = _PyInterpreterState_GET();
         switch (TYPE(n)) {
         case INDENT:
             interp->parser.listnode.level++;