[XFS] fix NULL pointer dereference in pagebuf

SGI Modid: 2.5.x-xfs:slinx:130709a

fs/xfs/pagebuf/page_buf.c

@@ -571,6 +571,7 @@ _pagebuf_lookup_pages(
 
 	if (!pb->pb_locked) {
 		for (pi = 0; pi < page_count; pi++) {
+			if (pb->pb_pages[pi])
 				unlock_page(pb->pb_pages[pi]);
 		}
 	}