Unrefrigerated Mashed Bananas: Are they safe to bake with?
I hate wasting, so I get why you are asking this, lol! 

I agree with "osp"... SMELL it. Sour or fishy or not like banana, well, then your banana bread might taste like something sour or fishy or not like banana.

Easiest solution is to just scoop off the top layer of the mashed banana that was exposed to the air!