convert-tpr charge zeroing broken

The charge-zeroing -zeroq function of convert-tpr is broken since ec5d2383 (relevant change here vs previous version here), which dates from 2008.

The index used in the zeroq() function originates from get_index, and so it is a global atom index, not moltype-aware. This worked before because the old t_topology has a all-concatenated array for atoms.

Thus this causes array out-of-bound write, or incorrect atom charge-zeroing. (Confirmed with ASAN and gdb)

Attached is a tpr and index file. For reproduction, gmx convert-tpr -s em.tpr -o em_out.tpr -zeroq -n index.ndx with group 12 (Water) selected will segfault.

Should this particular function of the tool be removed?