Fix the compile error on mac for memcheck mode

This new member does not seem useful. 
You assert in the new destructor code that m_tableAllocationSize - m_tupleLength * m_tuplesPerBlock == 0.
If the assert is true, you can use (m_tupleLength * m_tuplesPerBlock) eveywhere instead of m_tableAllocationSize -- I don't expect that to cause a performance issue.
This allows us to use the same TupleBlock structure unconditionally in all compiles, which improves readability.