DEB-6586 SlickGrid Checkbox plugin: Fixed an exception thrown in handleKeyDown()

@sergiopereiraTT: Code review please.

#### Fixed following exception.
`Uncaught TypeError: Cannot read property 'id' of undefined slick.checkboxselectcolumn.js?t=1403543665:63`
#### Call-stack:

`handleKeyDown 					slick.checkboxselectcolumn.js?t=1403543665:63`
`Event.notify   				slick.core.js?t=1403543665:126`
`trigger 						slick.grid.js?t=1403543665:1008`
`handleKeyDown 					slick.grid.js?t=1403543665:2177`
`jQuery.event.dispatch 			jquery.js?t=1403543665:5095`
`$event.dispatch 				jquery.event.drag.js?t=1403543665:374`
`jQuery.event.add.elemData.handle 	jquery.js?t=1403543665:4766`

#### Issue:
Issue happens when some columns are remove from the grid and then spacebar is pressed in the grid. Checkbox-plugin uses spacebar to toggle the checkbox state. Grid passes the last active cell as argument to the `handleKeyDown()`, which might no longer be a valid cell. Ideally, Grid should update its active-cell if the columns are reset. But , anyway, this fix will safeguard us from the exception.