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 
So having the object itself as hostname is automatically reused by the SFTP Stream ?
Can you add a code comment for that ?
It took me a while to understand and I can imagine a major confusion for other people seeing this.