Core - App Crashes When Evaluating Expressions with a Leading Operator

[EmmmaTech]

This issue was found when testing for another similar issue, #11.

If you enter an expression like 0 + or 2 * the app will crash. This is most likely because the program is trying to access a value in the value stack that doesn't exist. This means that it will cause a buffer overflow and crash.

A solution to fix this includes checking if there are enough values and operators in the stack, and otherwise warn the user that this expression is invalid.

[r1cola]

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