msgstore: fetch missing headers in visual mode

fetches missing headers when in visual selection mode. This prevents
large archive operations from panicking due to a nil pointer
dereference.

The archive command will return an error to the ui when a nil message is
encountered to signal that the message store is not ready yet.

Signed-off-by: Koni Marti <koni.marti@gmail.com>

commands/util.go

@@ -188,6 +188,9 @@ func MsgInfoFromUids(store *lib.MessageStore, uids []uint32) ([]*models.MessageI
 		if !ok {
 			return nil, fmt.Errorf("uid not found")
 		}
+		if infos[i] == nil {
+			return nil, fmt.Errorf("message store not ready yet")
+		}
 	}
 	return infos, nil
 }