[#162] netmap: Fix possible panic

Placement policy is unvalidated external input.
Under no circumstances should we panic here.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

netmap/netmap.go

@@ -258,6 +258,9 @@ func (m NetMap) ContainerNodes(p PlacementPolicy, pivot []byte) ([][]NodeInfo, e
 		}
 
 		if p.unique {
+			if c.processedSelectors[sName] == nil {
+				return nil, fmt.Errorf("selector not found: '%s'", sName)
+			}
 			nodes, err := c.getSelection(*c.processedSelectors[sName])
 			if err != nil {
 				return nil, err