msub_check_for_release(): Initialize msc_role_a_c

This fixes the following compiler error:

msub.c: In function ‘msub_fsm_active’:
msub.c:85:35: error: ‘msc_role_a_c’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
   || (msc_role_a_c && msc_role_a_c->ran->type == OSMO_RAT_EUTRAN_SGS)))
                       ~~~~~~~~~~~~^~~~~
msub.c:59:26: note: ‘msc_role_a_c’ was declared here
  struct msc_role_common *msc_role_a_c;
                          ^~~~~~~~~~~~

Change-Id: Id518dea77d01ed0518ca7cba6b1b363f1c8e6543
1 file changed