DatabaseComponent.setSeen() should call setStatusSeenIfVisible().

Otherwise an exception may be thrown if an offered message is no
longer in the DB.

components/net/sf/briar/db/DatabaseComponentImpl.java

@@ -1163,7 +1163,7 @@ DatabaseCleaner.Callback {
 						T txn = db.startTransaction();
 						try {
 							for(MessageId m : seen) {
-								db.setStatus(txn, c, m, Status.SEEN);
+								db.setStatusSeenIfVisible(txn, c, m);
 							}
 							db.commitTransaction(txn);
 						} catch(DbException e) {