Fix infinite loop ( grothoff, please check !)

git-svn-id: https://gnunet.org/svn/gnunet@34247 140774ce-b5e7-0310-ab8b-a85725594a96
Don't destroy channel if it was already destroyed.

git-svn-id: https://gnunet.org/svn/gnunet@34248 140774ce-b5e7-0310-ab8b-a85725594a96