WIP: fix crash in layout computation when changing all nodes of a zone to gateway mode

This change is probably not a proper fix, somebody with more expertise on
this code should look at it.

Here is how to reproduce the crash:

- start with a layout with two zones
- move all nodes of a zone to gateway mode: `garage layout assign fea54bcc081f318 -g`
- `garage layout show` will panic with a backtrace

Fortunately, the crash is only on the RPC client side, not on the Garage
server itself, and `garage layout revert` still works to go back to the
previous state.

As far as I can tell, this bug is present since Garage 0.9.0 which
includes the new layout assignation algorithm:

  #296

src/rpc/layout/version.rs

@@ -650,8 +650,11 @@ impl LayoutVersion {
 		let mut cost = CostFunction::new();
 		for (p, assoc_p) in prev_assign.iter().enumerate() {
 			for n in assoc_p.iter() {
-				let node_zone = zone_to_id[self.expect_get_node_zone(&self.node_id_vec[*n])];
-				cost.insert((Vertex::PZ(p, node_zone), Vertex::N(*n)), -1);
+				if let Some(&node_zone) =
+					zone_to_id.get(self.expect_get_node_zone(&self.node_id_vec[*n]))
+				{
+					cost.insert((Vertex::PZ(p, node_zone), Vertex::N(*n)), -1);
+				}
 			}
 		}
 
@@ -751,8 +754,11 @@ impl LayoutVersion {
 					if let Some(prev_assign) = prev_assign_opt {
 						let mut old_zones_of_p = Vec::<usize>::new();
 						for n in prev_assign[p].iter() {
-							old_zones_of_p
-								.push(zone_to_id[self.expect_get_node_zone(&self.node_id_vec[*n])]);
+							if let Some(&zone_id) =
+								zone_to_id.get(self.expect_get_node_zone(&self.node_id_vec[*n]))
+							{
+								old_zones_of_p.push(zone_id);
+							}
 						}
 						if !old_zones_of_p.contains(&z) {
 							new_partitions_zone[z] += 1;