Qt4: Avoid a potential division by zero

Fixes an annoying crash for me.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/util/input_slider.cpp

@@ -303,8 +303,10 @@ void SeekSlider::mouseMoveEvent( QMouseEvent *event )
 
         QPoint target( event->globalX() - ( event->x() - posX ),
                   QWidget::mapToGlobal( QPoint( 0, 0 ) ).y() );
-        secstotimestr( psz_length, ( ( posX - margin ) * inputLength ) / ( size().width() - handleLength() ) );
-        mTimeTooltip->setTip( target, psz_length, chapterLabel );
+        if( likely( size().width() > handleLength() ) ) {
+            secstotimestr( psz_length, ( ( posX - margin ) * inputLength ) / ( size().width() - handleLength() ) );
+            mTimeTooltip->setTip( target, psz_length, chapterLabel );
+        }
     }
     event->accept();
 }