buf_page_get_known_nowait(): Relax a bogus debug assertion.

When mode==BUF_KEEP_OLD, buffered inserts are being merged to the page.
It is possible that a read request for a page was pending while the page
was freed in DROP INDEX or DROP TABLE. In these cases, it is OK (although
useless) to merge the buffered changes to the freed page.

storage/innodb_plugin/buf/buf0buf.c

@@ -2148,7 +2148,7 @@ buf_page_get_known_nowait(
 	ut_a(buf_block_get_state(block) == BUF_BLOCK_FILE_PAGE);
 #endif /* UNIV_DEBUG || UNIV_BUF_DEBUG */
 #if defined UNIV_DEBUG_FILE_ACCESSES || defined UNIV_DEBUG
-	ut_a(block->page.file_page_was_freed == FALSE);
+	ut_a(mode == BUF_KEEP_OLD || !block->page.file_page_was_freed);
 #endif
 
 #ifdef UNIV_IBUF_COUNT_DEBUG