Force a null value for Instructions that are dominated by the active
Instruction.

In seec::trace::FunctionState (the recreated state of a function call), when
calling getCurrentRuntimeValue(), if the index of the requested Instruction
is higher than the index of the currently active Instruction, return a nullptr
regardless of the current runtime value of the Instruction.

This effectively handles issue 11: https://github.com/mheinsen/seec/issues/11