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[C++] Bound put<X> var data against the remaining buffer so a uint64 length cannot wrap the position - #1184
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…length cannot wrap the position. The check must stay a subtraction: pos + length wraps for a uint64 length and passes sbePosition().
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With a uint64 var data length, put(src, length) advanced the position with sbePosition(pos + length), which can wrap, pass the bounds check and let the memcpy write past the buffer.
put(src, length) now compares length with bufferLength - position, which cannot wrap, before advancing. The std::string and std::string_view overloads delegate to it. This is the encode-side counterpart of #1175.
Part of #969.
Verified: VarDataPutUint64LengthTest fails on master (negative-size-param in putText() under ASan) and passes here; sbe-tool checkstyle and test, cppbuild and cppbuild --debug-build --sanitise-build pass.