libosmogsm/gsup.c: fix error code in osmo_gsup_encode()

Missing (unset) type of to be encoded message is not a memory
allocation failure (-ENOMEM), this is definitely a mistake.

Change-Id: Ibbac18e2b68b765c17c2bc959c4c085037953a7f
diff --git a/src/gsm/gsup.c b/src/gsm/gsup.c
index 9c2f817..18b3580 100644
--- a/src/gsm/gsup.c
+++ b/src/gsm/gsup.c
@@ -535,7 +535,7 @@
 
 	/* generic part */
 	if(!gsup_msg->message_type)
-		return -ENOMEM;
+		return -EINVAL;
 
 	msgb_v_put(msg, gsup_msg->message_type);