Detect over/under-flow in UnsizedArray::operator[]

Was causing bad substitutions in mort table because of WordOffsetToIndex()
producing negative numbers that were cast to unsigned int and returned as
large numbers (which was desirable, so they would be rejected), but then
they were cast to int when passed to this operator and acting as small
negative integers, which was bad...

Detect overflow.  Ouch, however, now I see this still fails on 32-bit.
Guess I'm going to revert an earlier change.
1 file changed