net: handle undefined parent in _unrefTimer and _destroy

[haggholm]

The fix and approach are from #64491 by @Shivay-98; opening from a fresh branch to land it, since that PR stalled awaiting the requested changes (unrelated commit dropped, failing test addressed, Signed-off-by added).

Socket.prototype._unrefTimer and Socket.prototype._destroy both walk the socket _parent chain with a strict !== null check. When teardown leaves a socket's _parent as undefined (for example a TLS socket layered over another stream), the loop steps onto undefined and throws

TypeError: Cannot read properties of undefined (reading 'Symbol(timeout)')
    at TLSSocket._unrefTimer (node:net)
    at TLSWrap.onStreamRead (node:internal/stream_base_commons)

from an uncaught I/O callback, crashing the process. Switching both loops to a nullish != null check stops the walk on undefined as well.

The original PR fixed only _unrefTimer; its regression test also calls destroy(), which hits the same unpatched loop in _destroy, so that path is fixed and covered here too.

Fixes: #64490
Refs: #64491

Checklist

• make -j4 test (UNIX) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[nodejs-github-bot]

Review requested:

• @nodejs/net

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.14%. Comparing base (ed63b19) to head (6b91d30).
⚠️ Report is 56 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64644      +/-   ##
==========================================
- Coverage   92.01%   90.14%   -1.88%     
==========================================
  Files         381      741     +360     
  Lines      170167   242095   +71928     
  Branches    26075    45552   +19477     
==========================================
+ Hits       156585   218239   +61654     
- Misses      13291    15364    +2073     
- Partials      291     8492    +8201     

Files with missing lines	Coverage Δ
lib/net.js	94.37% <100.00%> (+3.37%)	⬆️

... and 514 files with indirect coverage changes

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