[#115] netmap: Fix type getters

* Add type instance check for nil to avoid panic by
  accessing fields.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

netmap/types.go

@@ -335,6 +335,10 @@ func (p *PlacementPolicy) SetContainerBackupFactor(backupFactor uint32) {
 }
 
 func (p *PlacementPolicy) GetReplicas() []Replica {
+	if p == nil {
+		return nil
+	}
+
 	return p.replicas
 }