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[C++] Check that the var data length prefix fits in the buffer before reading it - #1174
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… reading it. <x>Length(), skip<X>() and <x>() read the length prefix at the current position, so the check must come before the read and not only on the position computed from it.
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Length(), skip() and () read the var data length prefix before checking that it fits in the buffer, so a frame truncated inside the prefix makes them read up to sizeof(length) bytes past the end; Length() returns those bytes as the length. get() and getAsString() already advanced the position over the prefix first.
The three accessors now throw E100 when the prefix does not fit, before reading it.
Part of #969.
Verified: VarDataLengthPrefixBoundsTest fails on master (heap-buffer-overflow in payloadLength() under ASan) and passes here; sbe-tool checkstyle and test, cppbuild and cppbuild --debug-build --sanitise-build pass.