storage: return error when tombstone on new generation

It is not allowed to put tombstone on an empty generation.

storage/index.go

@@ -115,8 +115,7 @@ func (ti *treeIndex) Tombstone(key []byte, rev revision) error {
 	}
 
 	ki := item.(*keyIndex)
-	ki.tombstone(rev.main, rev.sub)
-	return nil
+	return ki.tombstone(rev.main, rev.sub)
 }
 
 func (ti *treeIndex) Compact(rev int64) map[revision]struct{} {

storage/key_index.go

@@ -90,12 +90,17 @@ func (ki *keyIndex) restore(created, modified revision, ver int64) {
 
 // tombstone puts a revision, pointing to a tombstone, to the keyIndex.
 // It also creates a new empty generation in the keyIndex.
-func (ki *keyIndex) tombstone(main int64, sub int64) {
+// It returns ErrRevisionNotFound when tombstone on an empty generation.
+func (ki *keyIndex) tombstone(main int64, sub int64) error {
 	if ki.isEmpty() {
 		log.Panicf("store.keyindex: unexpected tombstone on empty keyIndex %s", string(ki.key))
 	}
+	if ki.generations[len(ki.generations)-1].isEmpty() {
+		return ErrRevisionNotFound
+	}
 	ki.put(main, sub)
 	ki.generations = append(ki.generations, generation{})
+	return nil
 }
 
 // get gets the modified, created revision and version of the key that satisfies the given atRev.