ARG: remove test against error_ptr(-1) that is historical value, no longer returned by libjson-c >= 0.11 (#5755)

git-svn-id: https://svn.osgeo.org/gdal/trunk@28021 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_vc12