Fix for fparser optimization with NaN
Fix for fparser optimization with NaN

This probably isn't the only place where the optimizer is implicitly
assuming that values aren't NaN, but it's the only one I could find
and it's sufficient to fix my test case.

Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh