Fix: Postfix evaluation final_result null pointer dereference (#871)

[Akshatshukla-25]

Description

This PR addresses Bug 4 of issue #871. It adds validation to ensure that the output pointer parameter final_result is not NULL inside evaluate_postfix_expr in src/expression_evaluation/postfix_evaluation.c before storing the final calculation result.

Resolves

• Resolves part of Bug: Potential segmentation faults and crashes due to missing NULL checks and out-of-bound array writes #871 (Bug 4)

Changes

• postfix_evaluation.c: Added final_result == NULL validation inside evaluate_postfix_expr.
• Makefile: Linked missing help objects to resolve test suite build errors (carried over to this branch).

Verification & Testing

• Ran make fmt to ensure style consistency.
• Compiled successfully via make.
• Ran and passed all unit tests via make test.