Slight performance enhancement to HyperlinkGetter.
The gecko vbuf (and probably any future callers) only call HyperlinkGetter::next() when they encounter an embedded object character.
That is, if there are no embedded object characters, there will be no calls.
Thus:
1. For IAccessibleHypertext, we don't need to call nHyperlinks. hyperlink will fail or return null if we pass an invalid index anyway.
2. For IAccessibleHypertext2, we can lazily fetch the hyperlinks on the first call to get(), thus saving a potential cross-process call when there are none.
