Get a couple more LIBMESH_PETSC_NULLPTR

I was hoping to cherry-pick all of these fixes, but we've been taking
too long to get v1.8 out and there are a number of conflicts.

First bugfix release for 1.7.0.

Tag v1.7.0.

Tag third release candidate for the 1.7 series.

Tag second release candidate for the 1.7 series.

Tag first release candidate for the 1.7 series.

Second bugfix release for 1.6.0.

First bugfix release for 1.6.0.

Tag v1.6.0.

Tag third release candidate for v1.6.0.