Issue #19437: Fix convert_op_cmp() of decimal.Decimal rich comparator, handle

PyObject_IsInstance() failure

Modules/_decimal/_decimal.c

@@ -3009,7 +3009,13 @@ convert_op_cmp(PyObject **vcmp, PyObject **wcmp, PyObject *v, PyObject *w,
             *wcmp = Py_NotImplemented;
         }
     }
-    else if (PyObject_IsInstance(w, Rational)) {
+    else {
+        int is_instance = PyObject_IsInstance(w, Rational);
+        if (is_instance < 0) {
+            *wcmp = NULL;
+            return 0;
+        }
+        if (is_instance) {
             *wcmp = numerator_as_decimal(w, context);
             if (*wcmp && !mpd_isspecial(MPD(v))) {
                 *vcmp = multiply_by_denominator(v, w, context);
@@ -3022,6 +3028,7 @@ convert_op_cmp(PyObject **vcmp, PyObject **wcmp, PyObject *v, PyObject *w,
             Py_INCREF(Py_NotImplemented);
             *wcmp = Py_NotImplemented;
         }
+    }
 
     if (*wcmp == NULL || *wcmp == Py_NotImplemented) {
         return 0;