Fix memory leak in LLVMFuzzerTestOneInput when Initialize() fails

[Copilot]

This PR fixes a memory leak in the LLVMFuzzerTestOneInput function in src/tools/spin/spinquic.cpp.

Issue:
When FuzzData->Initialize() returns false, the function exits early without deallocating the FuzzData object that was allocated with new, causing a memory leak on every such input.

Root Cause:

FuzzData = new FuzzingData(data, size);  // Line 1602: Memory allocated
if (!FuzzData->Initialize()) {
    return 0;  // Line 1604: Early return without cleanup - LEAK!
}

Fix:
Added delete FuzzData; before the early return to ensure proper cleanup:

FuzzData = new FuzzingData(data, size);
if (!FuzzData->Initialize()) {
    delete FuzzData;  // Added: Properly clean up allocated memory
    return 0;
}

Verification:

• ✅ Build passes successfully with the fix
• ✅ Pattern is consistent with existing code (normal execution path also calls delete FuzzData; at the end)
• ✅ Minimal change (1 line added) that precisely addresses the issue
• ✅ Compared with similar fuzzing code in recvfuzz.cpp which doesn't have this issue due to simpler constructor pattern

Fixes #5201.

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[guhetier]

[BVT / BVT (Debug, linux, ubuntu-22.04, x64, quictls, -Sanitize, -Test) (pull_request)](https://github.com/microsoft/msquic/actions/runs/15979720574/job/45073476333?pr=5202)Failing after 54m

This test seems to have consistently regressed recently. It isn't specific to this PR though.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.23%. Comparing base (c69379c) to head (1fe3bce).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5202      +/-   ##
==========================================
+ Coverage   85.53%   86.23%   +0.69%     
==========================================
  Files          59       59              
  Lines       18330    18330              
==========================================
+ Hits        15679    15807     +128     
+ Misses       2651     2523     -128     

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