Qt: don't allow negative screen-numbers

Close #13643

(cherry picked from commit cd112b85939b04f50983b1f2a705c448eb79b527)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/controller.cpp

@@ -953,7 +953,7 @@ void FullscreenControllerWidget::toggleFullwidth()
 
 int FullscreenControllerWidget::targetScreen()
 {
-    if( i_screennumber == -1 || i_screennumber > QApplication::desktop()->numScreens() )
+    if( i_screennumber < 0 || i_screennumber > QApplication::desktop()->numScreens() )
         return QApplication::desktop()->screenNumber( p_intf->p_sys->p_mi );
     return i_screennumber;
 }