Fix a logic error where calling uv_fs_event_stop() from the event
callback tripped on a `handle->dir_handle != INVALID_HANDLE_VALUE`
assert in uv_fs_event_queue_readdirchanges().
Fixes: https://github.com/libuv/libuv/issues/3258
PR-URL: https://github.com/libuv/libuv/pull/3259
Reviewed-By: Jameson Nash <vtjnash@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>