[REVIEW-3.5] crasher fdo#45987

Hi
somewhat of a containing fix ( I think there is other rework needed, see 
below ) that avoids the mentioned crasher, please consider 
cherry-picking to 3.5

Always ( afaict ) the code expects the index of the entry in the 
(maTabs) vector to correspond a tab of the same index. However the 
DeleteTab routine patched above will erase the entry for the tab but if 
that tab isn't the last tab but instead some random tab in the middle 
won't the order of the tabs be screwed ? We could check and only delete 
an entry if it is the last entry ( and otherwise make a null entry for 
the deleted tab ) but then we definitely would expect the code should be 
ready to deal with such a 'hole' (representing a sheet that no longer 
exists ) in the vector, that doesn't appear to be the case. Is this how 
it should work ? I could rework it like that if that is the intention, 
is it ?