In check_factor, check membership of the factor itself, not alpha
In check_factor, check membership of the factor itself, not alpha

Rationale: alpha is computed from public data, and should already be a
group member by construction. The factor itself may come from
untrusted input.

glondu/belenios,glondu/belenios,glondu/belenios,glondu/belenios,glondu/belenios