libssh2: fix crash in keyboard callback

- Always set the libssh2 'abstract' user-pointer to the libcurl easy
  handle associated with the ssh session, so it is always passed to the
  ssh keyboard callback.

Prior to this change and since 8b5f100 (precedes curl 8.0.0), if libcurl
was built without CURL_DEBUG then it could crash during the ssh auth
phase due to a null dereference in the ssh keyboard callback.

Reported-by: Andreas Falkenhahn

Fixes https://github.com/curl/curl/pull/11024
Closes https://github.com/curl/curl/pull/11026

lib/vssh/libssh2.c

@@ -3274,7 +3274,7 @@ static CURLcode ssh_connect(struct Curl_easy *data, bool *done)
                                               my_libssh2_free,
                                               my_libssh2_realloc, data);
 #else
-  sshc->ssh_session = libssh2_session_init();
+  sshc->ssh_session = libssh2_session_init_ex(NULL, NULL, NULL, data);
 #endif
   if(!sshc->ssh_session) {
     failf(data, "Failure initialising ssh session");