server.DeleteMessage catches channel or client==nil

Fixes #2020

Calling /msg histserv delete #wrongchan <msgid> returns spurrious success
message. Calling /msg histserv delete <randomchars> <msgid> returns
similar message.

These would lead to the pointer hist == nil, and
server.historyDB.DeleteMsgid() would return nil.

Returns errNoSuchChannel or errInvalidTarget if channel or client == nil

irc/errors.go

@@ -54,6 +54,7 @@ var (
 	errConfusableIdentifier           = errors.New("This identifier is confusable with one already in use")
 	errInsufficientPrivs              = errors.New("Insufficient privileges")
 	errInvalidUsername                = errors.New("Invalid username")
+	errInvalidTarget                  = errors.New("Invalid target")
 	errFeatureDisabled                = errors.New(`That feature is disabled`)
 	errBanned                         = errors.New("IP or nickmask banned")
 	errInvalidParams                  = utils.ErrInvalidParams

irc/server.go

@@ -1061,13 +1061,17 @@ func (server *Server) DeleteMessage(target, msgid, accountName string) (err erro
 				if status, _, _ := channel.historyStatus(config); status == HistoryEphemeral {
 					hist = &channel.history
 				}
+			} else {
+				return errNoSuchChannel
 			}
 		} else {
 			client := server.clients.Get(target)
 			if client != nil {
 				if status, _ := client.historyStatus(config); status == HistoryEphemeral {
 					hist = &client.history
 				}
+			} else {
+				return errInvalidTarget
 			}
 		}
 	}