[1.5.x] Fixed #19382 -- Stopped smtp backend raising exception when already closed

Thanks Sebastian Noack for the report and the initial patch.
Backport of ffa50ca35 from master.

django/core/mail/backends/smtp.py

@@ -60,6 +60,8 @@ class EmailBackend(BaseEmailBackend):
 
     def close(self):
         """Closes the connection to the email server."""
+        if self.connection is None:
+            return
         try:
             try:
                 self.connection.quit()