multibuttonentry: fix Segfault on pressing Delete Key

multibuttonentry: fix Segfault on pressing Delete Key

Summary:
Pressing "Delete" Key in MBE layout causes Segfault as MBE item is not deleted properly and also focus is not transferred back to MBE.

@fix

Signed-off-by: divyesh purohit <div.purohit@samsung.com>
Test Plan: Run MultibuttonEntry example from elementart_test . Add some MBE items, now press the delete key, it should delete items without causing segfaults and transfer focus to previous MBE item.

Reviewers: raster, cedric, CHAN, shilpasingh

Reviewed By: shilpasingh

Subscribers: rajeshps, govi

Differential Revision: https://phab.enlightenment.org/D3635

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>