As others saying, stop worrying as these horror stories are rare. But as I also experienced a similar thing in the past, I also used to be careful as even if it's rare, maybe you can be the exception that with this "horror story" may happen. So here are few ideas...
There are dozen of methods. My favorite, add fail safes into your story. Plus, make a short advertisement for it via the net. i.e. my novel is known already by many and it's still not being published due to some reworks, but they can give me a solid back up to prove I'm the one who created my MS. People whose read your MS can prove you was the one who created it. Plus I used to sign a non-confidentiality agreement with every reader which also can be used as a proof. Fail safes are also good, where you're adding something into the MS which can prove it's yours.
My primary example: In the past someone stolen one of my MS, tried to sell it, but then she failed with one of the fail safes as there were two words, Aurora Dynamics, which is sounded as a fictive company in the story, but actually it was the name of my company that about only few knew (Including the person that to she tried to sell my MS.). I used to add tricky descriptions into the story which is always proving I was the one who created it (i.e. vital descriptions which seems average, but they have double meaning.).
Other versions... I used to register all my works via Writer's Guild of America. I always have a script for the novels, so I can do that as they're cross protections. But this is working only in that case if you have script. Or other version... always keep your letter exchanges regarding your novel. I have more then 3000 letters about my present project what I've changed with every people whoever got in touch with my fantasy WIP. Same goes for novel websites... IP logs. Always store them. That's a useful backup. Another version... small advertisement via the net, while you're not giving out any vital info regarding your novel. In that case the title is already connecting to you, that novel is already connecting to you. i.e. if you're writing into the google... "Crystal Shade", "Crystal Shade Angeni" or simply "Angeni", one of the first three hits are already belongs to my novel's website. Everyone can look after it, everyone can connect my name to it.
Your story can be safe, if you're making useful fail safes and make your work safe. Otherwise... be careful as I also faced with similar things, but I did against it and the one who tried to steal it failed greatly thanks to these fail safes. This thing is happening rarely, but it's better to be prepared for it, instead of leaving your MS vulnerable.