Fix: Assertion Failure on Open Slot Dialog when Deleting a MultiSlot Link [5988]
Bug Number: 5988
Release notes (y/n): Yes
For Release Nums: 7.1.5

During the processing of MULTISLOT_LINKS_CHANGED callback in the Open Slot 
Dialog (SlotQtDlg class), some computations updating the internal cell 
selection state are attempting to access the MultiSlot's deleted SubSlot 
(Link).  Before, this condition was being detected at a low level at which 
an invalid subslot index is regarded as a serious error (assertion failure).

This condition is now being detected at a somewhat higher level, in RootColRef
::compositeSlot() without failing an assertion.  Testing confirmed that this 
new check is positive in the bug scenario (during the transition of deleting 
a link), but not in other typical related processing. (This outcome is 
expected, otherwise we would be seeing this assertion failure in other 
circumstances).