Wrap indoor USB adapter into plastic bag for outdoor usage?
You say the adapter is behind the light shroud. I wouldn't be worried about that part. Besides you will have to poke a hole in the plastic bag for the plug and the cable thereby making it no longer moisture proof. Adapters and cables are fairly cheap and it should last for a while depending on your weather.
If the adapter itself shorts internally (which is not likely considering where you put it, and the fact that it is sealed) it could trip your breaker but probably won't draw enough current to melt or cause a fire. 


I would be more concerned with the cable to the camera and having moisture (maybe fog?) condense on the cable and drip into the camera. If you can rotate  the camera and have the cable enter from the bottom then any moisture will drip off the cable before it gets to the connection with the camera.

Good Luck!