Don't try to open visibilitymap when analyzing a foreign table

It's harmless, visibilitymap_count() returns 0 if the file doesn't
exist. But it's also very pointless. I noticed this when I added an
assertion in smgropen() that the relnumber is valid.

Discussion: https://www.postgresql.org/message-id/621a52fd-3cd8-4f5d-a561-d510b853bbaf@iki.fi

src/backend/storage/smgr/smgr.c

@@ -153,6 +153,8 @@ smgropen(RelFileLocator rlocator, BackendId backend)
 	SMgrRelation reln;
 	bool		found;
 
+	Assert(RelFileNumberIsValid(rlocator.relNumber));
+
 	if (SMgrRelationHash == NULL)
 	{
 		/* First time through: initialize the hash table */