[FD] libtiff: invalid write (CVE-2015-7554)

`_TIFFVGetField()' in libtiff-4.0.6 may write field data for certain extension tags to invalid or possibly arbitrary memory. Each tag has a `field_passcount' variable in their TIFFField struct: tiff-4.0.6/libtiff/tif_dir.h #276..289: ,-

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