player/player.go: ExecuteCommand: use strings.CutPrefix to make sure …

…the command starts with "/". (#988)

* player/player.go: add empty string check in command resolution.

* fix: player/player.go: use strings.CutPrefix.

server/player/player.go

@@ -330,10 +330,16 @@ func (p *Player) ExecuteCommand(commandLine string) {
 		return
 	}
 	args := strings.Split(commandLine, " ")
-	command, ok := cmd.ByAlias(args[0][1:])
+
+	name, ok := strings.CutPrefix(args[0], "/")
+	if !ok {
+		return
+	}
+
+	command, ok := cmd.ByAlias(name)
 	if !ok {
 		o := &cmd.Output{}
-		o.Errort(cmd.MessageUnknown, args[0])
+		o.Errort(cmd.MessageUnknown, name)
 		p.SendCommandOutput(o)
 		return
 	}