Ensure max width and height are initialized

Prevents X11 mode from doing very weird things when we are unable
to get the max window size from the window manager.
Found by scan-build

src/conio/x_events.c

@@ -1226,7 +1226,8 @@ static void resize_window()
 
 static void init_mode_internal(int mode)
 {
-	int mw, mh;
+	int mw = 0;
+	int mh = 0;
 
 	x11_get_maxsize(&mw, &mh);
 	free_last();