-Wzero-as-null-pointer-constant leads to compiler error in g++

[joernschellhaas]

As of boost 1.72, I'm getting "error: zero as null pointer constant" when using e.g. boost::chrono::milliseconds. This can IMO be fixed by using nullptr instead.

I've spotted the following places, but there might be more:

• duration.hpp:458
• duration.hpp:481
• duration_put.hpp:216
• time_point.hpp:185

[viboes]

Boost is compatible with C++98.

A PR is more than welcome if you manage with this fact.

[joernschellhaas]

Seems that NULL can be used to ensure compatibility. Not sure if I'll create a PR.