Merge pull request #3156 from weaveworks/fix-panic-maybe

Add null check to fix querier panic

report/node_set.go

@@ -74,6 +74,12 @@ func (n NodeSet) Delete(ids ...string) NodeSet {
 
 // UnsafeMerge combines the two NodeSets, altering n
 func (n *NodeSet) UnsafeMerge(other NodeSet) {
+	if other.psMap == nil {
+		return
+	}
+	if n.psMap == nil {
+		n.psMap = ps.NewMap()
+	}
 	other.psMap.ForEach(func(key string, otherVal interface{}) {
 		n.psMap = n.psMap.UnsafeMutableSet(key, otherVal)
 	})