Webpage crashes if invalid maths expression is input

[JoshdRod]

e.g: Inputting "10x +" into answer box and pressing enter will cause site to crash.
This is because the evaluation subroutines are expecting valid maths expressions, and are probably looping forever.
Solution: Add an expression validator that runs before evaluation.

[JoshdRod]

Should also check for empty strings:
#4 (comment)