fix: avoid uninitialized read in queue_using_linkedlist dequeue()

[Saadanjum0]

Description of Change

dequeue() in data_structures/queue_using_linkedlist.cpp declared int ele; without initializing it. When the queue is empty, the function only prints Stack is empty and falls straight through to return ele;, returning an undefined value instead of whatever is intended for the empty-queue case. This is undefined behavior, confirmed directly by clang:

warning: variable 'ele' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]

Because it's undefined behavior, the observed symptom depends on the compiler and optimization level rather than being one fixed value. At -O0 the uninitialized read reliably surfaces as a crash: the project's existing assert-based self-test style exercises dequeue() on an empty queue, and the pre-fix code aborts (SIGABRT) 5/5 runs when built with clang++ -std=c++17 -Wall -Wextra -O0. At -O2, the same pre-fix binary happened to pass 5/5 runs instead, because the optimizer picked 0 for the uninitialized value in that build — a coincidence of this compiler/flags, not a guarantee. That compiler/flag-dependent split is exactly why this is a real bug worth fixing rather than a false alarm: relying on an uninitialized read means the behavior can silently change with the toolchain.

Fix

• Initialize ele = 0 so the empty-queue case has a defined return value, matching the sentinel already used by queue_using_array.cpp's dequeue() for the same situation.
• Added braces around the previously brace-less if/else for clarity.
• Added a tests() self-test (assert-based, matches the project's existing test style) that exercises enqueue/dequeue ordering and asserts the empty-queue return value both before any enqueue and after draining the queue. With the fix applied, the test passes consistently at both -O0 and -O2.

Checklist

• Added description of change
• File name matches guidelines (no new file added)
• Added tests and example, test passes
• Relevant documentation/comments added
• PR title follows semantic commit guidelines
• Searched for duplicate/prior work on this bug (none found)
• I acknowledge that all my contributions will be made under the project's license.