Cleanup across the codebase: use nullptr instead of NULL or 0 in pointer comparison

[tautschnig]

This is really just a C++11 cleanup that should happen at some point; I've got a patch somewhere that replaces all uses of NULL by 0 - I should be able to tweak that to do nullptr instead.

[tautschnig]

Closing as there are very few occurrences left, and cleanup can be done as needed.