mncc_sock: Fix potential segfault in case MNCC app dies

We create a loop by not setting trans->callref = 0 before calling
trans_free(), as the latter would again send a MNCC_REL_IND up
the stack.

Also: Fix memory leak in case we try to read from mncc_sock
but socket is just gone.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index 9ab8840..138cdfc 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -336,8 +336,10 @@
 	LOGP(DCC, LOGL_NOTICE, "Clearing all currently active transactions!!!\n");
 
 	llist_for_each_entry_safe(trans, temp, &net->trans_list, entry) {
-		if (trans->protocol == protocol)
+		if (trans->protocol == protocol) {
+			trans->callref = 0;
 			trans_free(trans);
+		}
 	}
 }