Add assertion in expand_vacuum_rel() for non-autovacuum path

The code path where the assertion is added helps to check that
autovacuum always includes a relation OID when doing a vacuum on it.
Extracted from a larger patch set to add support for SKIP LOCKED with
manual VACUUM commands.

Author: Nathan Bossart
Discussion: https://postgr.es/m/9EF7EBE4-720D-4CF1-9D0E-4403D7E92990@amazon.com

src/backend/commands/vacuum.c

@@ -443,6 +443,12 @@ expand_vacuum_rel(VacuumRelation *vrel)
 		Form_pg_class classForm;
 		bool		include_parts;
 
+		/*
+		 * Since autovacuum workers supply OIDs when calling vacuum(), no
+		 * autovacuum worker should reach this code.
+		 */
+		Assert(!IsAutoVacuumWorkerProcess());
+
 		/*
 		 * We transiently take AccessShareLock to protect the syscache lookup
 		 * below, as well as find_all_inheritors's expectation that the caller