Fix compilation problem

Apparently while my compiler accepts the code

  std::queue q;
  q = {};

To clear a queue from all it's elements, the compiler on Jenkins
refuses this code, claiming that std::queue's constructor is explicit.

So rewrite this code in a different way which will hopefully make Jenkins
happy.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>