fix use-after-free: the call to mesh_channel_destroy will ultimately free 'op', so we must not update 'op' after the call; however, it is a good idea to clean up the 'channel' field just before the call (#3164)

git-svn-id: https://gnunet.org/svn/gnunet@31175 140774ce-b5e7-0310-ab8b-a85725594a96
-fix trailing spaces

git-svn-id: https://gnunet.org/svn/gnunet@31176 140774ce-b5e7-0310-ab8b-a85725594a96
mark timeout task as done if it is run, fixes #3171

git-svn-id: https://gnunet.org/svn/gnunet@31177 140774ce-b5e7-0310-ab8b-a85725594a96
-doxygen and formatting and style fixes

git-svn-id: https://gnunet.org/svn/gnunet@31178 140774ce-b5e7-0310-ab8b-a85725594a96