fix a crash when SAMODE'ing in a channel you're not joined to

irc/channel.go

 // ClientIsAtLeast returns whether the client has at least the given channel privilege.
 func (channel *Channel) ClientIsAtLeast(client *Client, permission modes.Mode) bool {
 	channel.stateMutex.RLock()
-	defer channel.stateMutex.RUnlock()
-
 	clientModes := channel.members[client]
+	channel.stateMutex.RUnlock()
+
+	if clientModes == nil {
+		return false
+	}
 
 	for _, mode := range modes.ChannelUserModes {
 		if clientModes.HasMode(mode) {