i#2049 burst_threads crash: remove invalid assert
i#2049 burst_threads crash: remove invalid assert

Removes an invalid assert that by code inspection can fire spuriously.
Discovered while analyzing i#2049.

Review-URL: https://codereview.appspot.com/312980043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio