Pass existing Net_SFTP object into Net_SFTP_Stream.

Otherwise a new SFTP connection is created by the Stream Wrapper (with no public host key verification).

Follows #9718
Needs backport to stable7
stable6 is not affected (not using SFTP Stream Wrapper)
Ref https://github.com/phpseclib/phpseclib/blob/0.3.6/phpseclib/Net/SFTP/Stream.php#L173

@PVince81 @LukasReschke @icewind1991 @blizzz 
Yes that will do, so all the explanations are in one place.