Fix server crash when revoking a corrupt bbssig Group.
Fix server crash when revoking a corrupt bbssig Group.

If an account was created with an invalid bbssig Group then the server
would hit a nil pointer if that user attempted to file a revocation.

Fixes #37.

sillvan/pond,ioerror/pond,ArneBab/pond,ArneBab/pond,agl/pond,saltthefries/pond,infinity0/pond,agl/pond,berkus/pond,infinity0/pond,sillvan/pond,ioerror/pond,berkus/pond,saltthefries/pond