gh-132835: Add defensive NULL checks to MRO resolution (GH-134763)

Currently, there are a few places where tp_mro could theoretically
become NULL, but do not in practice. This commit adds defensive checks for
NULL values to ensure that any changes do not introduce a crash and that
state invariants are upheld.

The assertions added in this commit are all instances where a NULL value would get passed to something not expecting a NULL, so it is better to catch an assertion failure than crash later on.

There are a few cases where it is OK for the return of lookup_tp_mro to be NULL, such as when passed to is_subtype_with_mro, which handles this explicitly.

Objects/typeobject.c

@@ -1780,7 +1780,7 @@ static int recurse_down_subclasses(PyTypeObject *type, PyObject *name,
 // Compute tp_mro for this type and all of its subclasses.  This
 // is called after __bases__ is assigned to an existing type.
 static int
-mro_hierarchy(PyTypeObject *type, PyObject *temp)
+mro_hierarchy_for_complete_type(PyTypeObject *type, PyObject *temp)
 {
     ASSERT_TYPE_LOCK_HELD();
 
@@ -1791,6 +1791,7 @@ mro_hierarchy(PyTypeObject *type, PyObject *temp)
         return res;
     }
     PyObject *new_mro = lookup_tp_mro(type);
+    assert(new_mro != NULL);
 
     PyObject *tuple;
     if (old_mro != NULL) {
@@ -1835,7 +1836,7 @@ mro_hierarchy(PyTypeObject *type, PyObject *temp)
         Py_ssize_t n = PyList_GET_SIZE(subclasses);
         for (Py_ssize_t i = 0; i < n; i++) {
             PyTypeObject *subclass = _PyType_CAST(PyList_GET_ITEM(subclasses, i));
-            res = mro_hierarchy(subclass, temp);
+            res = mro_hierarchy_for_complete_type(subclass, temp);
             if (res < 0) {
                 break;
             }
@@ -1926,7 +1927,7 @@ type_set_bases_unlocked(PyTypeObject *type, PyObject *new_bases, PyTypeObject *b
     if (temp == NULL) {
         goto bail;
     }
-    if (mro_hierarchy(type, temp) < 0) {
+    if (mro_hierarchy_for_complete_type(type, temp) < 0) {
         goto undo;
     }
     Py_DECREF(temp);
@@ -3432,6 +3433,7 @@ mro_implementation_unlocked(PyTypeObject *type)
          */
         PyTypeObject *base = _PyType_CAST(PyTuple_GET_ITEM(bases, 0));
         PyObject *base_mro = lookup_tp_mro(base);
+        assert(base_mro != NULL);
         Py_ssize_t k = PyTuple_GET_SIZE(base_mro);
         PyObject *result = PyTuple_New(k + 1);
         if (result == NULL) {
@@ -3466,9 +3468,12 @@ mro_implementation_unlocked(PyTypeObject *type)
         return NULL;
     }
 
+    PyObject *mro_to_merge;
     for (Py_ssize_t i = 0; i < n; i++) {
         PyTypeObject *base = _PyType_CAST(PyTuple_GET_ITEM(bases, i));
-        to_merge[i] = lookup_tp_mro(base);
+        mro_to_merge = lookup_tp_mro(base);
+        assert(mro_to_merge != NULL);
+        to_merge[i] = mro_to_merge;
     }
     to_merge[n] = bases;
 
@@ -8998,6 +9003,7 @@ type_ready_inherit(PyTypeObject *type)
 
     // Inherit slots
     PyObject *mro = lookup_tp_mro(type);
+    assert(mro != NULL);
     Py_ssize_t n = PyTuple_GET_SIZE(mro);
     for (Py_ssize_t i = 1; i < n; i++) {
         PyObject *b = PyTuple_GET_ITEM(mro, i);