Invalidate smgr_targblock in smgrrelease().

In rare circumstances involving relfilenode reuse, it might have been
possible for smgr_targblock to finish up pointing past the end.

Oversight in b74e94dc.  Back-patch to 15.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA%2BhUKGJ8NTvqLHz6dqbQnt2c8XCki4r2QvXjBQcXpVwxTY_pvA%40mail.gmail.com

src/backend/storage/smgr/smgr.c

@@ -296,6 +296,7 @@ smgrrelease(SMgrRelation reln)
 		smgrsw[reln->smgr_which].smgr_close(reln, forknum);
 		reln->smgr_cached_nblocks[forknum] = InvalidBlockNumber;
 	}
+	reln->smgr_targblock = InvalidBlockNumber;
 }
 
 /*