try to work around maybe-uninitialized warning

Author: ngoldbaum

tests/test_pythoncapi_compat_cext.c

@@ -1436,7 +1436,7 @@ test_long_api(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
     PyLongWriter_Discard(writer);
 
     writer = PyLongWriter_Create(1, 1, (void**)&digits);
-    digits[0] = 123;
+    *digits = 123;
     obj = PyLongWriter_Finish(writer);
 
     check_int(obj, -123);