Do not crash on add_key/reject_key if the previous one set the drop file. (#27796)

The umask set does not allow salt-key to overwrite the .dfn file. However, if the
master has not yet rotated the key but the rotation was requested, there should be no
reason to request a second time.

salt/crypt.py

@@ -55,6 +55,9 @@ def dropfile(cachedir, user=None):
     mask = os.umask(191)
     try:
         log.info('Rotating AES key')
+        if os.path.isfile(dfn):
+            log.info('AES key rotation already requested')
+            return
 
         with salt.utils.fopen(dfn, 'wb+') as fp_:
             fp_.write('')