Qdyn6 doesn't fail on NaN

[mpurg]

Qdyn6 compiled with optimizations doesn't fail on T==NaN and terminates normally.
Not urgent, but could be annoying.

I tried only with gcc4.8.4 but it seems like it's more general:
https://stackoverflow.com/questions/15944614/is-it-possible-to-make-isnan-work-in-gfortran-o3-ffast-math

A test case (timestep of 10fs): asd.zip

[acmnpv]

Strange error, but I think I saw something like this before and didn't think about it.
I guess the only way to fix it is to include the extra routine compiled without the optimization. I'll have a look into this.