do_steep segfaults with non-interacting system - Redmine #2917

At https://gerrit.gromacs.org/\#/c/9382/3 I accidentally tested something with steepest descent on a system that does not interact. In such cases, the max force is zero, and minimizers should exit gracefully rather than choose a step size based on e.g. dividing by the max force.

(from redmine: issue id 2917, created on 2019-04-03 by mark.j.abraham, closed on 2019-05-03)

• Changesets:
  • Revision f895f84a by Paul Bauer on 2019-05-02T07:54:20Z:

Fix FPE in do_steep

Fixes error when running non-interacting system in steepest decent
minimization.

Fixes #2917

Change-Id: Iaff794ab6c2d41e8c4b3d89feeafae6ae2372ddf