Fix: Deque accessors and display function null pointer dereference (#871)

[Akshatshukla-25]

Description

This PR addresses Bug 3 of issue #871. It adds explicit NULL pointer checks for stored elements inside the deque accessors (deque_get_front and deque_get_rear) and the output formatter (display_deque) in src/data_structures/deque.c to prevent crashes when NULL values are stored in the queue.

Resolves

• Resolves part of Bug: Potential segmentation faults and crashes due to missing NULL checks and out-of-bound array writes #871 (Bug 3)

Changes

• deque.c: Added dq->arr[...] == NULL checks in deque_get_front, deque_get_rear, and display_deque.
• Makefile: Linked missing help objects to resolve test suite build errors (carried over to this branch).

Verification & Testing

• Ran make fmt to format all code.
• Successfully compiled using make.
• Passed all unit tests using make test.