Don't keep running doEvent if MGED_STATE is NULL

Chris ran into a crash in MGED where doEvent was trying to access
something invalid in the course of shutting down.  We tried to
de-register the doEvent callback but that doesn't seem to be effective.
Testing a global flag did work, but Chris spotted that we could just
look for a NULL MGED_STATE instead since we do explicitly NULL that out
in mged_finish.

src/mged/doevent.c

@@ -119,7 +119,7 @@ doEvent(ClientData clientData, XEvent *eventPtr)
     struct mged_dm *save_dm_list;
     int status;
 
-    if (eventPtr->type == DestroyNotify || (unsigned long)eventPtr->xany.window == 0)
+    if (eventPtr->type == DestroyNotify || (unsigned long)eventPtr->xany.window == 0 || !MGED_STATE)
 	return TCL_OK;
 
     // The set_curr_dm logic here appears to be important for Multipane mode -