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
diff --git a/src/libmsc/msub.c b/src/libmsc/msub.c
index 4900732..2021ed8 100644
--- a/src/libmsc/msub.c
+++ b/src/libmsc/msub.c
@@ -56,7 +56,7 @@
 static void msub_check_for_release(struct osmo_fsm_inst *fi)
 {
 	struct msub *msub = fi->priv;
-	struct msc_role_common *msc_role_a_c;
+	struct msc_role_common *msc_role_a_c = NULL;
 	enum msc_role role_idx;
 	int role_present[MSC_ROLES_COUNT] = {};
 	struct osmo_fsm_inst *child;