Fixed positionAll(). It didn't clear nextpos1/nextpos2.

positionAll() doesn't clear nextpos1/nextpos2 when PNotify.notices is empty.
So, there is a bug in the following cases:
1. Includes pnotify.core.js only.
2. Call "new PNotify({ type: 'error', title: 'Title', text: 'Message' });"
3. Waits until all notification are closed.
4. Call "new PNotify({ type: 'error', title: 'Title', text: 'Message' });" again.