fix crash that occurs when dir is nil

builder/vmware/common/artifact.go

@@ -49,7 +49,10 @@ func (a *artifact) State(name string) interface{} {
 }
 
 func (a *artifact) Destroy() error {
-	return a.dir.RemoveAll()
+	if a.dir != nil {
+		return a.dir.RemoveAll()
+	}
+	return nil
 }
 
 func NewArtifact(remoteType string, format string, exportOutputPath string, vmName string, skipExport bool, keepRegistered bool, state multistep.StateBag) (packer.Artifact, error) {