What is stopping you from providing Malindo Air with the flight number (DAC-KUL OD 161/163/165 etc.) and finding out if they will allow you bring it onboard the aircraft?

Or just go to http://www.malindoair.com/faqs and search for "guitar" where it's stated that "small instruments e.g. a guitar, violin, saxophone, these items can be taken on board as hand carry (subject to hand carry limitations on size and weight)".  It's really not difficult to query the airlines directly and find out. 

And in a worst case scenario, during checking in and being issued the boarding pass, the attendant could request that you purchase another seat for the guitar (very unlikely, but possible) or request that you check the guitar in with your luggage (more likely).  In which case, either have a reliable guitar flight case on standby, or have someone take your guitar back home.  Usually most airlines will allow you to bring your guitar in a gig bag (soft case) into the cabin.

You can also opt to dismantle the guitar neck from the body and bring it in your bag; have an appropriate short screwdriver handy for this eventuality.

​​Please ​​provide ​​us with your​​ Malindo Air confirmation code (6 characters) or your e-ticket n​​umber (13 digit ​​starts with 816) in order for us to further assist you in your issue​​.​​

(subject to hand carry limitations on size and weight)

I saw a guy carrying his guitar case on my flight