add missing return if exception oocurs

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxManagerImpl.java

@@ -156,6 +156,7 @@ public class MailboxManagerImpl implements MailboxManager {
 			} catch (IOException | DbException e) {
 				logException(LOG, WARNING, e);
 				disposeConnection(true);
+				return;
 			}
 
 			connectionRegistry