src: remove TOCTOU race condition when encoding SAB-backed Buffers

[aduh95]

If the BufferView is backed by a SharedArrayBuffer, we want to copy the content before passing it to String::Encode in case the data is mutated in a different thread.

Otherwise the validate_utf8 and convert_valid_utf8_to_utf16 might get bytes, leading to undefined behavior

node/src/string_bytes.cc

Lines 585 to 600 in e209dbe

	if (buflen >= 32 && simdutf::validate_utf8(buf, buflen)) {
	// We know that we are non-ASCII (and are unlikely Latin1), use 2-byte
	// In the most likely case of valid UTF-8, we can use this fast impl
	// For very short input, it is slower, so we limit min size
	size_t u16size = simdutf::utf16_length_from_utf8(buf, buflen);
	if (u16size > static_cast<size_t>(v8::String::kMaxLength)) {
	isolate->ThrowException(ERR_STRING_TOO_LONG(isolate));
	return MaybeLocal<Value>();
	}
	return EncodeTwoByteString(
	isolate, u16size, [buf, buflen, u16size](uint16_t* dst) {
	size_t written = simdutf::convert_valid_utf8_to_utf16(
	buf, buflen, reinterpret_cast<char16_t*>(dst));
	CHECK_EQ(written, u16size);
	});
	}

Refs: https://hackerone.com/reports/3752489

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.13%. Comparing base (ca37665) to head (dc2be31).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_buffer.cc	60.00%	0 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63517      +/-   ##
==========================================
+ Coverage   90.12%   90.13%   +0.01%     
==========================================
  Files         718      718              
  Lines      227911   228418     +507     
  Branches    42813    42938     +125     
==========================================
+ Hits       205404   205889     +485     
+ Misses      14288    14273      -15     
- Partials     8219     8256      +37     

Files with missing lines	Coverage Δ
src/encoding_binding.cc	54.08% <100.00%> (+1.16%)	⬆️
src/node_buffer.cc	68.64% <60.00%> (+0.17%)	⬆️

... and 85 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73642/