fix: clear vlr_subscr->msc_conn_ref when the conn is discarded

Before this, it was for example possible to crash the MSC by the vty 'show
subscriber' command, which would dereference a potentially stale
vsub->msc_conn_ref pointer.

Related: OS#3050
Change-Id: Ia4105d9f135ba3216ad3c86157be7658b1d568fb
1 file changed