Fix unintentional fallthrough

When LEAST_BEHIND_MASTER routing criteria was used, the info level logging
function would fall through to the default case. In debug builds, this
would trigger a debug assertion.

server/modules/routing/readwritesplit/rwsplit_select_backends.cc

@@ -231,6 +231,8 @@ static void log_server_connections(select_criteria_t criteria, const SRWBackendL
             MXS_INFO("replication lag : %d in \t[%s]:%d %s",
                      b->server->rlag, b->server->name,
                      b->server->port, STRSRVSTATUS(b->server));
+            break;
+
         default:
             ss_dassert(!true);
             break;