fixed issue when ")" is not present

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

@@ -97,7 +97,7 @@ this.singlefile.lib.vendor.cssMinifier = this.singlefile.lib.vendor.cssMinifier
 
 			let foundTerminator = false, endIndex = pattern.lastIndex - 1;
 
-			while (foundTerminator === false && endIndex + 1 <= maxIndex) {
+			while (foundTerminator === false && endIndex + 1 <= maxIndex && endIndex != -1) {
 				endIndex = css.indexOf(terminator, endIndex + 1);
 
 				// endIndex == 0 doesn't really apply here