Adds cleanup method

Author: msutovsky-r7

modules/exploits/linux/http/pivotx_index_php_overwrite.rb

@@ -144,13 +144,18 @@ def restore
     vprint_error('Failed to restore original content') unless res&.code == 200 && res.body.include?('Wrote contents to file index.php')
   end
 
+  def cleanup
+    super
+    # original content can be any string, it cannot be nil
+    restore if @original_value.nil?
+  end
+
   def exploit
     vprint_status('Logging in PivotX')
     login
     vprint_status('Modifying file and injecting payload')
     modify_file
     vprint_status('Triggering payload')
     trigger_payload
-    restore
   end
 end