Don’t let bug reports be your specification.
Spend more time in the backlog and make sure you have work ready before the beginning of a Sprint. A great practice worth starting ASAP is called “Backlog Grooming”
Find more about “Backlog Grooming” in the Scrum Guide
Additional great Tips on grooming the backlog in Roman Pichler’s blog
Why does Grooming Matter?
http://www.romanpichler.com/blog/product-backlog/grooming-the-product-backlog/