Correct the error handling in TInterpreter::Execute(...,TObjArray*)

The issues at http://root.cern.ch/phpBB3/viewtopic.php?f=3&t=16769
was actually about not handling correctly defaulted parameters
and not about the 'this pointer'.

In addition LastIndex for a TObjArray is not as reliable as for
TList to get the number of entries in the collection (i.e. it
was wrong to use it here).