qlibssh2 crashes when multiple Ssh2Client are created and destroy

For example:
```
{
SSh2Client client1 (settings)
// ... Do stuff with client1
}

{
Ssh2Client client2 (settings)
// ... Do stuff with client2, this will crash on windows
}
```

Client2 will crash because libssh2_exit() and then libssh2_init() is called again. libssh2_init() should only be called once, and libssh2_exit() should be called on program exit.