Fix slave reconnection logic

Allowing calls to select_connect_backend_servers even when all slaves are
connected solves the debug assertion in select_connect_backend_servers
that happens when the execution of a queued query causes a new connection
to be created.

maxscale-system-test/mxs1506_no_master.cpp

@@ -19,7 +19,7 @@ bool query(TestConnections& test)
     sleep(5);
     Row row = get_row(test.maxscales->conn_rwsplit[0], "SELECT @a");
     test.maxscales->disconnect();
-    return row[0] == "1";
+    return !row.empty() && row[0] == "1";
 }
 
 void block(TestConnections& test, std::vector<int> nodes)