Shapelib: add SHPSetFastModeReadObject() to avoid calloc() in SHPReadObject() (but only one SHPObject valid at a time) (derived from patch by Alvaro Huarte, #5272)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26622 f0d54148-0727-0410-94bb-9a71ac55c965