fix crashes due to dropping NULL links (fixes 8fdd4a5c50de8dcc2315492c65a2755c9485f6ca.pdf)
fix crashes due to dropping NULL links (fixes 8fdd4a5c50de8dcc2315492c65a2755c9485f6ca.pdf)

darcs-hash:20080417060709-d0692-520c66c39e3c9600776c7357a3d1f90540159465.gz

nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf