Do not try to destroy runner vm if it's already deleted

Vm can be deleted via UI or respirate might run label again. So if vm is
already deleted, we should not try to delete it. "vm" returns nil, and
fails with "NoMethodError".

prog/vm/github_runner.rb

@@ -184,9 +184,11 @@ def before_run
     rescue Octokit::NotFound
     end
 
-    vm.private_subnets.each { _1.incr_destroy }
-    vm.sshable.destroy
-    vm.incr_destroy
+    if vm
+      vm.private_subnets.each { _1.incr_destroy }
+      vm.sshable.destroy
+      vm.incr_destroy
+    end
 
     hop_wait_vm_destroy
   end

spec/prog/vm/github_runner_spec.rb

@@ -238,6 +238,18 @@
 
       expect { nx.destroy }.to hop("wait_vm_destroy")
     end
+
+    it "does not destroy vm if it's already destroyed" do
+      expect(nx).to receive(:register_deadline)
+      expect(nx).to receive(:decr_destroy)
+      expect(client).to receive(:get).and_raise(Octokit::NotFound)
+      expect(client).not_to receive(:delete)
+      expect(github_runner).to receive(:vm).and_return(nil)
+      expect(sshable).not_to receive(:destroy)
+      expect(vm).not_to receive(:incr_destroy)
+
+      expect { nx.destroy }.to hop("wait_vm_destroy")
+    end
   end
 
   describe "#wait_vm_destroy" do