fixed parsing of font property

lib/single-file/css-fonts-minifier.js

@@ -186,7 +186,7 @@ this.fontsMinifier = this.fontsMinifier || (() => {
 		}
 		const font = declarations.children.filter(node => node.property == "font").tail;
 		if (font) {
-			for (let node = font.data.value.children.tail; node && node.type != "WhiteSpace"; node = node.prev) {
+			for (let node = font.data.value.children.tail; node && node.data.type != "WhiteSpace"; node = node.prev) {
 				if (node.data.type == "String" || node.data.type == "Identifier") {
 					fontFamilyNames.push(getFontFamily(cssTree.generate(node.data)));
 				}