[poppler] Unicode page labels should not contain a NUL at the end

[Fabio D'Urso]

The attached one-page document (generated with TexLive 2011 by an Okular user) 
labels its only page with the unicode string "1" <FEFF0031>.
In qt4, Page::label() returns a QString containing "1\0" (i.e. 2 character 
long with a NUL character at the end). I haven't tested, but I think the same 
goes for the other frontends.

I think that the issue is in PageLabelInfo::indexToLabel, because it's here 
where the extra NUL is appended.
This extra NUL seems to be wrong, because GooStrings are not NUL terminated in 
the non-unicode case.
It also confuses Okular, making it assume that page labels differ from page 
indices.

I'm attaching a patch that removes the NUL at the end.
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