Fix heap-buffer-overflow in ModbusLayer on truncated packets#2159
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Signed-off-by: Sai Asish Y <say.apm35@gmail.com>
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Fixes #2155
A TCP packet to port 502 with fewer than 8 payload bytes was still parsed as a ModbusLayer, so the header getters (getTransactionId, getLength, toString, etc.) read past the end of the buffer. This adds a ModbusLayer::isDataValid check on the payload length before constructing the layer, matching how GtpV2Layer and BgpLayer guard their parsing. Added a regression test with a truncated Modbus packet that fails without the guard.