[[ Bug 13554 ]] Corruption of stacks

This is a little murky as written - I suggest adding a method to MCObjectOutputStream which is 'MeasureStringRefNew' - this should do the same as WriteStringRefNew except that it returns the length of the thing being written rather than writing it. You can then refactor this code to simply be:
`
t_size += 1 + 1 + 4 + MeasureStringRefNew(stack, MCstackfileversion >= 7000) + MeasureStringRefNew(kMCEmptyString, MCstackfileversion >= 7000);