Internal libtiff: in TIFFDefaultDirectory(), reset any already existing extented tags installed by user code through the extender mechaninm before calling the extender callback (#5054)

git-svn-id: https://svn.osgeo.org/gdal/trunk@28236 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_vc12