Protect self.links for multiple additions of links.

When calling `-addLinkToURL:withRange:` multiple times, only the last add
is maintained. The underlaying cause is that `self.links` array is reset
when `self.attributedText` is set in the method's `if (attributes)` body.
The alternative fix was to protect the `self.links` property in the
`-setAttributedText:` method with the following: 

    self.links = self.links ?: [NSArray array];

I felt the guard and replace strategy was less invasive.

Anon,
Andrew