Incorporate fixes made in GDAL

[atlight]

The GDAL/OGR copy of libopencad has a number of fixes which ought to be downstreamed into this library. Many of them are crash fixes, and because GDAL is part of Google's oss-fuzz we are likely to continue to be alerted to similar issues.

See https://github.com/OSGeo/gdal/commits/master/gdal/ogr/ogrsf_frmts/cad/libopencad

[sandyre]

I would like to fetch them into the repository, but have a pretty much lack of time. There is https://github.com/nextgis-borsch/lib_opencad, seems like it contains latest fixes. But since it's not a fork, fixes have to be taken manually.

I am working on whole library refactoring (in /dev branch, very unstable and possible not compiable at all) to make it much simplier to maintain and improve.