common/AsyncReserver.h: use empty() instead of size()

common/AsyncReserver.h: use empty() instead of size()

Fix warning for usage of queue.size() in do_queues(). Use empty()
since it should be prefered as it has, following the standard, a
constant time complexity regardless of the containter type. The
same is not guaranteed for size().

warning from cppchecker was:
[common/AsyncReserver.h:40]: (performance) Possible inefficient
  checking for 'queue' emptiness.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>