Algorithm/ants: repilication bugfix

Fixed a bug when a replication can rarely be created on a storage where
it couldn't be placed

Signed-off-by: Artem Plakunov <artacc@lvk.cs.msu.su>
Algorithm/ants, Algorithm/randomalg: fixed a crash if there are no requests

Signed-off-by: Artem Plakunov <artacc@lvk.cs.msu.su>