Fix entropy calculation in gmx anaeig

Fix entropy calculation in gmx anaeig

gmx anaeig reads eigenvectors produced by gmx covar and
can compute entropy according to Schlitter's formula or
based on the quasiharmonic method. If the number of
eigenvectors is not consistent with the number of atoms
the entropy calculation could use uninitialized variables.
Added a warning when this happens.

Fixes #2668

Change-Id: I4f265212ce0a7bf82e25e1aa6f9cbb544c45db3f