Fixes a complicated bug in g_anaeig.

If the number of frames for a covariance analysis is fewer than the
number of degrees of freedom, g_covar would happily print nonsense
eigenvalues in the eigenval.xvg file. This would then lead g_anaeig
to give NaN entropy values. By limiting the number of output lines in
the eigenvalue file this should be resolved. In addition a warning is
printed.

Change-Id: I01693a0fa9f3ba5b5784543a04d0d88b33a755c2