VNC may segfault during the connection process

Guacamole's VNC implementation assumes that the rfbClient object will be available at the time that keyboard or mouse events occur, but this is not guaranteed. That object will only be available once the VNC connection process has completed.

Thus, if keys are pressed (or the clipboard contents are sent) while the connection is not yet ready, the event handler tries to send an event to NULL, a segfault occurs, and the Guacamole connection closes.