The condition contradicts the exception text:
            if (offset >= st.st_size) {
                PyErr_SetString(PyExc_ValueError,
                                "mmap offset is greater than file size");
                return NULL;
            }
The condition should be changed to (offset > st.st_size), similar to the later condition which is correct:
        } else if (offset + (size_t)map_size > st.st_size) {
            PyErr_SetString(PyExc_ValueError,
                            "mmap length is greater than file size");
            return NULL;
        }

The patch is attached.