fixes a segfault (crash) in these conditions:

* the form's AfterInsert StarBasic script does a reload() of the form

* the insert happens by clicking into a cell which has
  scripts linked to the events FocusLost and FocusGained.
  The case that led to discovering this involved a ListBox whose
  RecordSource changes when focus is gained or lost.

Then there was a race condition between the cell's dispose (from the
form's reload()) and the _old_ cell's FocusLost being executed.
NewStyleUNOScript::invoke would then call
xControl->getModel
with xControl being the involved cell,
leading FmXGridCell::getModel() to call m_pColumn->getModel()
with m_pColumn == NULL.

Change-Id: Ifb4402d37ee4faec80087ffccabe102acc016d60