Description of Change

The previous version of the code did not handle properly the incorrect inputs potentially leasing to segmentation faults. This change updates the code to handle all possible cases, i.e.:

• when there are not enough operands to pop from the stack (during the computation and at the end of it),
• when there are elements left on the stack after fishing the computation.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
Handle incorrect inputs in `postfix_evaluation`.