ASIO's socket_ops::sync_send must never return a negative value

socket_ops::sync_send captures information about errors in a std::error_code, and apparently ASIO doesn't check that value before it adds the return value (number of bytes sent) to its count of bytes sent already. If sync_send returns -1 (because of an error), it will end up adding that to its count and then resend a byte which corrupts the message.