Fix crash in res_xmpp when deleting pubsub node from CLI

An error existed in res_xmpp where it would attempt to delete attributes from
a node that itself was also deleted. Per the iksemel documentation, attributes
added using iks_insert are copied to the parent node's stack, and will be
reclaimed when that node is itself destroyed.

(closes issue ASTERISK-20982)
Reported by: marcelloceschia
patches:
  delete-node-fix.diff uploaded by marcelloceschia (License 6036)
........

Merged revisions 381159 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@381160 f38db490-d61c-443f-a65b-d21fe96a405b