Prevent NPE in HtmlAttributes.getIndex

Apparently it’s possible for a “mode” value to be such that
AttributeName.getQName(mode) returns null. So this change adds a null
check to catch that.

Otherwise, without this change, some documents cause that code to throw
a NullPointerException.

src/nu/validator/htmlparser/impl/HtmlAttributes.java

@@ -184,7 +184,8 @@ public AttributeName getAttributeNameNoBoundsCheck(int index) {
 
     public int getIndex(String qName) {
         for (int i = 0; i < length; i++) {
-            if (names[i].getQName(mode).equals(qName)) {
+            if (names[i].getQName(mode) != null &&
+                    names[i].getQName(mode).equals(qName)) {
                 return i;
             }
         }