libssh2: s/ssherr/sftperr/

The debug output used ssherr instead of sftperr which not only outputs
the wrong error code but also casues a warning on Windows.

Follow-up to 7370b4e39f

Reported-by: Gisle Vanem
Bug: 7370b4e39f (r41334700)
Closes #5799

lib/vssh/libssh2.c

@@ -1256,7 +1256,7 @@ static CURLcode ssh_statemach_act(struct connectdata *conn, bool *block)
           result = CURLE_SSH;
         sshc->actualcode = result;
         DEBUGF(infof(data, "error = %d makes libcurl = %d\n",
-                     ssherr, (int)result));
+                     sftperr, (int)result));
         state(conn, SSH_STOP);
         break;
       }