Set data pointer to NULL after an error; this keeps the mmap_dealloc()...

Set data pointer to NULL after an error; this keeps the mmap_dealloc() function from trying to do msync(-1);munmap(-1).
2.3 bugfix candidate, but this bug isn't critical enough that the fix has to go into 2.3.4

Modules/mmapmodule.c

@@ -912,6 +912,7 @@ new_mmap_object(PyObject *self, PyObject *args, PyObject *kwdict)
 			   prot, flags,
 			   fd, 0);
 	if (m_obj->data == (char *)-1) {
+	        m_obj->data = NULL;
 		Py_DECREF(m_obj);
 		PyErr_SetFromErrno(mmap_module_error);
 		return NULL;