When using conference mode DAHDI_CONF_DIGITALMON ensure conf_chan is set. Otherwise we dereference a NULL 'conf_chan'. Fixes regression introduced in revision 9381 Signed-off-by: Shaun Ruffell <sruffell@digium.com> git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/trunk@9457 a0bf4364-ded3-4de4-8d8a-66a801d63aff