Workaround for issue #3642. (#3670)

For some reason, there are nil values stored in the uidMatrix, which
results in a segmentation fault when trying to access the Uids field.
The root cause of this issue is still unclear but this change adds an
additional check to prevent the segmentation fault.

query/groupby.go

@@ -221,8 +221,9 @@ func (sg *SubGraph) formResult(ul *pb.List) (*groupResults, error) {
 				if algo.IndexOf(ul, srcUid) < 0 {
 					continue
 				}
+
 				ul := child.uidMatrix[i]
-				for _, uid := range ul.Uids {
+				for _, uid := range ul.GetUids() {
 					dedupMap.addValue(attr, types.Val{Tid: types.UidID, Value: uid}, srcUid)
 				}
 			}