Turn "check" into assertion

In commit 1813f951ca, I mistakenly assumed that one particular
function named CheckSomething() checks Something.  It doesn't, it merely
return the state to check.  Fix by adding the necessary Assert() call.

Good chance to add trailers for commit 1813f951ca:
Reviewed-by: Manuel Reyes Bravo <manuelreyesbravo@gmail.com>
Reviewed-by: Antonin Houska <ah@cybertec.at>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>

Backpatch-through: 19
Discussion: https://postgr.es/m/CAA-aLv5MF6BLL+BWvix2Yw+CBardtH43AofPReQunhDZPNBtuA@mail.gmail.com

src/backend/commands/repack.c

@@ -1455,8 +1455,8 @@ copy_table_data(Relation NewHeap, Relation OldHeap, Relation OldIndex,
 		if (!concurrent)
 			LockRelationOid(OldHeap->rd_rel->reltoastrelid, lmode);
 		else
-			CheckRelationOidLockedByMe(OldHeap->rd_rel->reltoastrelid,
-									   lmode, false);
+			Assert(CheckRelationOidLockedByMe(OldHeap->rd_rel->reltoastrelid,
+											  lmode, false));
 	}
 
 	/*