send_existing: return after send_existing
send_existing: return after send_existing

When a piece of data is sent from the send_existing() function we must
make the parent function return afterwards. Otherwise we risk that the
parent function tries to send more data and ends up getting an EGAIN for
that more data and since it can only return one return code it doesn't
return info for the successfully sent data.

As this change is a regression I now added a larger comment explaining
why it has to work like this.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2