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/mncc_sock.c b/openbsc/src/mncc_sock.c
index 3052009..e9e6276 100644
--- a/openbsc/src/mncc_sock.c
+++ b/openbsc/src/mncc_sock.c
@@ -144,6 +144,7 @@
 	return rc;
 
 close:
+	msgb_free(msg);
 	mncc_sock_close(state);
 	return -1;
 }