Fix segfault on PyPy3 when passing NULL method_name to _Pyx_dict_iterator

Cython/Utility/Optimize.c

@@ -294,8 +294,9 @@ static CYTHON_INLINE PyObject* __Pyx_dict_iterator(PyObject* iterable, int is_di
         // On PyPy3, we need to translate manually a few method names.
         // This logic is not needed on CPython thanks to the fast case above.
         static PyObject *py_items = NULL, *py_keys = NULL, *py_values = NULL;
-        const char *name = PyUnicode_AsUTF8(method_name);
         PyObject **pp = NULL;
+        if (method_name) {
+            const char *name = PyUnicode_AsUTF8(method_name);
             if (strcmp(name, "iteritems") == 0) pp = &py_items;
             else if (strcmp(name, "iterkeys") == 0) pp = &py_keys;
             else if (strcmp(name, "itervalues") == 0) pp = &py_values;
@@ -307,6 +308,7 @@ static CYTHON_INLINE PyObject* __Pyx_dict_iterator(PyObject* iterable, int is_di
                 }
                 method_name = *pp;
             }
+        }
 #endif
     }
     *p_orig_length = 0;