Fix vmware compact_disk step when there are no additional disks

builder/vmware/common/step_compact_disk.go

@@ -37,8 +37,8 @@ func (s StepCompactDisk) Run(state multistep.StateBag) multistep.StepAction {
 		return multistep.ActionHalt
 	}
 	
-	moreDisks := state.Get("additional_disk_paths").([]string)
-	if len(moreDisks) > 0 {
+	if state.Get("additional_disk_paths") != nil {
+		if moreDisks := state.Get("additional_disk_paths").([]string); len(moreDisks) > 0 {
 			for i, path := range moreDisks {
 				ui.Say(fmt.Sprintf("Compacting additional disk image %d",i+1))
 				if err := driver.CompactDisk(path); err != nil {
@@ -46,7 +46,7 @@ func (s StepCompactDisk) Run(state multistep.StateBag) multistep.StepAction {
 					return multistep.ActionHalt
 				}
 			}
-		
+		}
 	}
 
 	return multistep.ActionContinue