gsm48_rx_gmm_att_req: fix error handling: don't clean up NULL llme

Change-Id: I78903d0846568d4e98b6e43e62bbe3c329b5bc68
diff --git a/src/gprs/gprs_gmm.c b/src/gprs/gprs_gmm.c
index 8441f6b..f26c22b 100644
--- a/src/gprs/gprs_gmm.c
+++ b/src/gprs/gprs_gmm.c
@@ -1443,7 +1443,7 @@
 	rc = gsm48_tx_gmm_att_rej_oldmsg(msg, reject_cause);
 	if (ctx)
 		mm_ctx_cleanup_free(ctx, "GPRS ATTACH REJ");
-	else
+	else if (llme)
 		gprs_llgmm_unassign(llme);
 
 	return rc;