Fixed the crash for "4 -"
However, because of how RPN works, this will yield -4.
MathSys only knows to substitute the last answer when there are no more
values to pop off of the RPN.
There is no easy way to fix that because if you swap the values then "-
4" will become inverted.
There is no way to distinguish an input of "- 4" from "4 -" once it
leaves splitInput.