Fix illegal memory access in FE calculations

Fix illegal memory access in FE calculations

With free-energy calculations not using lambda states, an output
buffer would be accessed one element beyond it's allocated size.

Note that this code should be completely refactored, but not
in a release branch.

Fixes #3173

Change-Id: I677e602ba96c9f64fbf79a626e43c9e590c18bea