Fixed output of eigenvalues in g_covar. Fixes #1575

Commit 972032bfb8cd38 introduced a bug that would lead to eigenvalues
only written to .xvg file if "-last" is explicitly stated on the
command line. Otherwise no eigenvalues would appear in the .xvg file.
The eigenvalues are written in a loop from '0' to 'end', but since
'end' is initialized with '-1', the loop would never be executed.
This patch moves the code that computes 'end' one block upwards
before the output to file.

Change-Id: I738c9dd77ff9e6e2daae89b6d2063755dfba88af