All of my fingers can do that, Coolpink.  It can make things mildly annoying at first, but I can tell you from playing guitar that it *can* be an advantage later, since your fingers can a few tricks some people's can't. 

But to keep them from locking backwards, keep you fingers curled forwards a bit, and try to use a bit less pressure.  They only really lock if you're applying pressure over a certain amount.  If you press the fingertip lightly against the thumb of the other hand and go between bent backwards and bent forwards, you can learn how much pressure you have to apply before it locks.  It will almost certainly be less than you need to play the notes. 

For all that some people may tend to worry you about it, it is just your joints on those fingers are a bit more flexible than average.  I've known good players on assorted instruments whose fingers could bend either way, and it can be a slight advantage sometimes, if you work a bit to get control over it so it only happens when you do it on purpose.