Stolen ideas are a big concern for many beginning writers, but few people actually steal ideas because:
1) Most creative people are mainly interested in their own ideas,
2) The actual success of any idea is in the realization of it, not the concept,
3) It's not financially worth it to steal ideas, at least in countries with good copyright laws.
With all that said, if you publish a novel, the rights to the movie version may go to the publisher, depending on your contract. Or, if you sell the movie rights yourself, you will likely lose creative control once the sale is finalized. There have been any number of authors who were unhappy with the filmed versions of their books.
But on the other hand, it probably means MANY many more people will read your original book, even if it is very different from the final movie. So it's a trade-off. But is it better for no one to read you at all? You've written it --isn't it better to publish it than to lock it in a safe?