Fix syntax error in generated code found by pflua-quickcheck
Fix syntax error in generated code found by pflua-quickcheck

* src/pf/codegen.lua (filter_builder): It's not guaranteed that we're at
  the end of a block when returning true or false, when the continuation
  is ACCEPT or REJECT, so surround those returns in a do block.

mpeterv/pflua,SnabbCo/pflua