check for no screens before asserting

src/openbox.cc

@@ -153,13 +153,13 @@ Openbox::Openbox(int argc, char **argv)
     }
   }
 
-  assert(_focused_screen);
-
   if (_screens.empty()) {
     printf(_("No screens were found without a window manager. Exiting.\n"));
     ::exit(1);
   }
 
+  assert(_focused_screen);
+
   ScreenList::iterator it, end = _screens.end();
   
   // run the user's script or the system defaults if that fails