[fix] Membership check of the sender of a certification was buggy

- Cf: https://forum.duniter.org/t/bug-a-l-ajout-d-une-certification-d-un-non-membre-dans-bloc/5952
- Introduced in cf7d2068

app/lib/dal/fileDAL.ts

@@ -898,10 +898,10 @@ export class FileDAL {
   async isMember(pubkey:string):Promise<boolean> {
     try {
       const idty = await this.iindexDAL.getFromPubkey(pubkey);
-      if (idty === null) {
-        return false
+      if (idty && idty.member) {
+	  return true
       }
-      return true;
+      return false
     } catch (err) {
       return false;
     }