suzerain: enforce realness of res.Rz

Should be unnecessary, but there are small magnitude
real values showing up here.  Could fix this at the
inverse FFT level, but that gives me the willies at the moment.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@45524 0bd85f0b-c151-43b9-a1e9-27231104551f