Check for NaN when resolving link destinations

Merged Andreas Perstinger requested to merge andipersti/zathura-pdf-mupdf:fix_issue_33 into develop

Since version 1.21.0 of mupdf, link destinations that don't set a top and left coordinate will return NaN instead of 0 when they get resolved using fz_resolve_link(). This will mess up further calculations for the actual link location and all these links will then point to the last page of the document.

This will fix #33 (closed).

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