avoid use of groups_arr

src/Corpus.zig

@@ -127,9 +127,10 @@ pub fn init(
                 members[members.len - 1] = user_idx;
                 try user2groups[user_idx].append(allocator, @intCast(u32, i));
             } else {
+                const name = groups.get(i).name;
                 return err.returnf(
                     "user '{s}' not found, member of group '{s}'",
-                    .{ member_name, groups_arr[i].name },
+                    .{ member_name, name },
                     error.NotFound,
                 );
             }