prune correctly when series is both leaf and branch

-  this issue was discovered in discussions of issue #714
-  this also fixes #797

idx/memory/memory.go

@@ -1136,8 +1136,24 @@ func (m *MemoryIdx) Prune(orgId int, oldest time.Time) ([]idx.Archive, error) {
 			}
 
 			log.Debug("memory-idx: series %s for orgId:%d is stale. pruning it.", n.Path, org)
-			defs := m.delete(org, n, true)
-			statMetricsActive.Dec()
+			defs := make([]idx.Archive, 0)
+			if n.HasChildren() {
+				// this path is a leaf and branch node, so lets remove the defIds to make it just a branch.
+				for _, id := range n.Defs {
+					log.Debug("memory-idx: deleting %s from index", id)
+					defs = append(defs, *m.DefById[id])
+					delete(m.DefById, id)
+				}
+				n.Defs = []string{}
+				if tagSupport {
+					for _, def := range defs {
+						m.deindexTags(&(def.MetricDefinition))
+					}
+				}
+			} else {
+				defs = m.delete(org, n, true)
+			}
+			statMetricsActive.Add(-1 * len(defs))
 			pruned = append(pruned, defs...)
 			m.Unlock()
 		}