The check for nullptr before calling delete or delete[] is not necessary, because these operators check for nullptr already itself, as required by the C++ standard.

POV-Ray's code has a lot of (newer?) code that don't check, and some (older?) code that does these unecessary checks.

This pull request removes the unnecessary checks but keep them when there are more to do, e.g. joining threads or calling other functions before the object is deleted.