evas/callbacks: Prevent post_event_callback_call recursion.

evas/callbacks: Prevent post_event_callback_call recursion.

If this function is called recursively, it will free the list of post
callbacks before the list stops being used, which will cause a segfault.

The only place where this seems to happen is on
https://phab.enlightenment.org/T124, probably due to the extensive mouse
events which are launched in an unexpected way.

This bug started happening after commit
164cc07237395f8fe6efa465e4f0c0c4863f78ed, where the
_elm_scroll_page_x_get() started being called by a post_event callback,
and forcing an edje recalc. This recalc triggered another post_event
callback, thus causing the mentioned segfault.

If there's a better way to prevent this from happening, please change
the mentioned code.