Fix vmware compact_disk step when there are no additional disks

builder/vmware/common/step_compact_disk.go

@@ -37,16 +37,16 @@ func (s StepCompactDisk) Run(state multistep.StateBag) multistep.StepAction {
 		return multistep.ActionHalt
 	}
 	
-	moreDisks := state.Get("additional_disk_paths").([]string)
-	if 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 {
-				state.Put("error", fmt.Errorf("Error compacting additional disk %d: %s", i+1, err))
-				return multistep.ActionHalt
+	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 {
+					state.Put("error", fmt.Errorf("Error compacting additional disk %d: %s", i+1, err))
+					return multistep.ActionHalt
+				}
 			}
 		}
-		
 	}
 
 	return multistep.ActionContinue