It's not condescending at all if the person thinks they're helping, but it is almost always irritating. Some people just don't understand what it's like. Don't just get mad at them. Educate them. If someone cuts you off and finishes your word/sentence, ask them nicely to not do that, and let them know that you know they are trying to help, but it can be really frustrating for people who stutter to be cut off and "assisted" that way.