osmo-ns-dummy: don't leak primitive messages

The message in the primitive must be freed by the user

Change-Id: I2a7b19f019485c7b1f15baf20b62edc19b663415
diff --git a/utils/osmo-ns-dummy.c b/utils/osmo-ns-dummy.c
index a2070ca..58e8152 100644
--- a/utils/osmo-ns-dummy.c
+++ b/utils/osmo-ns-dummy.c
@@ -188,6 +188,9 @@
 /* called by the ns layer */
 int gprs_ns_prim_cb(struct osmo_prim_hdr *oph, void *ctx)
 {
+	if (oph->msg)
+		msgb_free(oph->msg);
+
 	return 0;
 }