Fix lib/libnv tests compilation with -std=c++11

[dim]

Summary

In this thread Oliver Hartmann describes an error he gets when compiling
the lib/libnv tests:

https://lists.freebsd.org/pipermail/freebsd-current/2015-March/054858.html

The errors are because the tests use the ATF_REQUIRE_EQ() macro with
NULL as its second argument, and the macro tries to output its arguments
on a std::ostringtream.

However, in C++11 you cannot output a plain NULL to any std::ostream;
you have to qualify the type of pointer by casting it. This is also
what ATF does internally for some of its tests.

I went through all the cases and added the correct casts, and now
lib/libnv can be successfully compiled with -std=c++11.

Test Plan

Compile and run tests.

[rstone]

Thanks for doing this.

[dim]

Closed by commit rS279760 (authored by @dim).