[poppler] Don't crash on malformed documents

[Kristian Høgsberg]

Albert Astals Cid wrote:
> http://research.sun.com/techrep/2005/smli_tr-2005-143.pdf has a link that 
> points nowhere (page 4, the link with the [3]) and the namedtree for finding 
> links is crashing on it, this patchs fixes the crash.

Oops, yeah, we need to check the return value from bsearch properly.
Thanks, slightly edited patch applied.
Kristian