That usually happens when you have the finger straight (or even bent backwards, perhaps) and you go to a forward "regular" curve.  Right? 

There's a couple things you can try/do.  First, don't let the finger go all the way straight or backwards so it "locks" in the first place.  Second, if you experiment a bit, for most folks it only "clicks" when you are using over a certain amount of pressure.  Experiment and find if maybe you don't need that much pressure on the pinky to play.

Finding out how little pressure you need to use will be helpful when you go to work on vibrato, since you really don't want the finger locking when you try to do that.  But you'll need to learn to use light pressure and keep the fingers curved for that anyway. 

So it is no big barrier to playing.  Doesn't hurt to have a doctor check it out, though, and make sure it isn't from an old injury or maybe early arthritis, both of which might affect finger mobility enough to affect playing.