fix findbugs warnings

Author: mikesamuel

src/main/java/org/owasp/html/CssTokens.java

@@ -103,6 +103,7 @@ public boolean hasNext() {
  }
 
  public String next() {
+ if (!hasToken()) { throw new NoSuchElementException(); }
  String token = token();
  advance();
  return token;