MDEV-29593 fixup: Avoid a leak if rseg.undo_cached is corrupted

trx_purge_truncate_rseg_history(): Avoid a leak similar to the one that was fixed in MDEV-31324, in case a supposedly cached undo log page is not found in the rseg.undo_cached list.

Author: dr-m

storage/innobase/trx/trx0purge.cc

@@ -510,9 +510,9 @@ trx_purge_truncate_rseg_history(trx_rseg_t& rseg,
  if (undo->hdr_page_no == hdr_addr.page)
  goto found_cached;
  ut_ad("inconsistent undo logs" == 0);
- break;
- found_cached:
- UT_LIST_REMOVE(rseg.undo_cached, undo);
+ if (false)
+  found_cached:
+  UT_LIST_REMOVE(rseg.undo_cached, undo);
  static_assert(FIL_NULL == 0xffffffff, "");
  if (UNIV_UNLIKELY(mach_read_from_4(TRX_RSEG + TRX_RSEG_FORMAT +
  rseg_hdr->page.frame)))