Qt: CoverArtLabel: fix sizing bug.

QPixmapCache can't compute using an undefined maxsize.
Introduced by a2314241

modules/gui/qt4/components/interface_widgets.cpp

@@ -541,7 +541,7 @@ void CoverArtLabel::showArtUpdate( const QString& url )
     QPixmap pix;
     if( !url.isEmpty() && pix.load( url ) )
     {
-        pix = pix.scaled( minimumWidth(), maximumHeight(),
+        pix = pix.scaled( minimumWidth(), minimumHeight(),
                           Qt::KeepAspectRatioByExpanding,
                           Qt::SmoothTransformation );
     }