Bugfix: Don’t crash anymore when moving focus to another screen (Thanks, Mirko!)

This fixes ticket #24. The crash appeared only under special conditions:
- The screen on which you switch to must be empty or at least not as
  full as the screen on which you were before.
- You switch focus using your mouse.
- You previously selected a column/row which is not availably on the
  destination screen (for example (1,1) and switching to an empty
  screen).

src/handlers.c

@@ -173,7 +173,11 @@ int handle_enter_notify(void *ignored, xcb_connection_t *conn, xcb_enter_notify_
                         LOG("ERROR: No such screen\n");
                         return 0;
                 }
+                c_ws->current_row = current_row;
+                c_ws->current_col = current_col;
                 c_ws = &workspaces[screen->current_workspace];
+                current_row = c_ws->current_row;
+                current_col = c_ws->current_col;
                 LOG("We're now on virtual screen number %d\n", screen->num);
                 return 1;
         }