timers: fix regression with clearImmediate()

This commit fixes a regression introduced in 0ed8839a that caused
additional queued immediate callbacks to be ignored if
`clearImmediate(immediate)` was called within the callback for
`immediate`.

PR-URL: https://github.com/nodejs/node/pull/9086
Fixes: https://github.com/nodejs/node/issues/9084


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>