gmx do_dssp doesn't work properly when CHARMM FFs are used

I was surprised about always seeing that the generated outputs were ignoring one residue of the chains... then I found it is a real bug: https://mailman-1.sys.kth.se/pipermail/gromacs.org_gmx-users/2017-August/115064.html

Sadly it's still valid with 2019.6 (I couldn't check with 2020.x as it seems to fail for me, maybe due to #3444 (closed) (they are systems prepared using charmm-gui). I couldn't find any opened bug for that issue... I am opening one then :/ sorry for the noise if it's a duplicated bug