Make something up. It's not like they're going to call your boss and say "Hey we're trying to steal this employee away from you, how much money are you paying him?"
Some exceptions to this: 1. You work a government job where your salary is public knowledge 2. You are unemployed and they can just call your previous employer. 3. You listed your current boss as a reference and they're probably getting a call anyway. 4. You expect that they're going to perform an extensive background check that might uncover your salary anyway.
