existing proton hunt tries to find H name that doesn't exist

Hi all, 
at https://github.com/project-gemmi/gemmi/blob/master/src/dssp.cpp#L119
```
    h_pos = donor->res->find_atom("H", '*', El::H)->pos;

```
returns a nullptr on ```donor->res->find_atom("H", '*', El::H)``` and will crash in the case of the n-terminal residue.  These have H1, H2 and/or H3 protons.

