Fix crash in Message Screensaver

In the Draw() method check to make sure that the view is valid, is
connected to the window, that the window is locked, that the screen
is valid and that the BBitmap buffer is valid. This (hopefully) fixes the
last of the bugs experienced due to #4260.

It is not sufficient to just check that the view is valid to prevent the crash,
you must also check that it is connected to a window via Window() and
that the window is locked.