pg_publication repetitious code

[Alvaro Herrera]

This very small patch removes some duplicated code in pg_publication.

-- 
Álvaro Herrera                            http://www.linkedin.com/in/alvherre

diff --git a/src/backend/catalog/pg_publication.c b/src/backend/catalog/pg_publication.c
index d442c8e0bb..347324f320 100644
--- a/src/backend/catalog/pg_publication.c
+++ b/src/backend/catalog/pg_publication.c
@@ -403,17 +403,9 @@ GetPublicationByName(const char *pubname, bool missing_ok)
 {
 	Oid			oid;
 
-	oid = GetSysCacheOid1(PUBLICATIONNAME, Anum_pg_publication_oid,
-						  CStringGetDatum(pubname));
-	if (!OidIsValid(oid))
-	{
-		if (missing_ok)
-			return NULL;
-
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("publication \"%s\" does not exist", pubname)));
-	}
+	oid = get_publication_oid(pubname, missing_ok);
+	if (!OidIsValid(oid) && missing_ok)
+		return NULL;
 
 	return GetPublication(oid);
 }