HPCC-19852 Check pointer expandedClusterGroup before using it

An unbound thor cluster causes esp to segfault when user clicks on
an ECLWatch link for more details. The segfault is due to invalid
expandedClusterGroup pointer returned by getGroupFromCluster().
This fix checks the pointer. If invalid, an exception will be
thrown.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>

dali/base/dadfs.cpp

@@ -9762,6 +9762,8 @@ static IGroup *getClusterNodeGroup(const char *clusterName, const char *type, bo
     Owned<IGroup> nodeGroup = queryNamedGroupStore().lookup(nodeGroupName);
     CInitGroups init(timems);
     Owned<IGroup> expandedClusterGroup = init.getGroupFromCluster(type, cluster, true);
+    if (!expandedClusterGroup)
+        throwStringExceptionV(0, "Failed to get group for '%s' cluster '%s'", type, clusterName);
     if (!expandedClusterGroup->equals(nodeGroup))
         throwStringExceptionV(0, "DFS cluster topology for '%s', does not match existing DFS group layout for group '%s'", clusterName, nodeGroupName.str());
     Owned<IGroup> clusterGroup = init.getGroupFromCluster(type, cluster, false);