The fix of MDEV-24569 and MDEV-24695 introduced a race condition when
a table is being rebuilt or dropped during the fseg_page_is_free() check.
The server would occasionally crash during the execution of the test
encryption.create_or_replace.

The fil_space_t::STOPPING flag can be set by DDL operations. Normally,
such concurrent operations are prevented by a metadata lock (MDL).
However, neither the change buffer merge nor the fil_crypt_thread() are
protected by MDL.

fil_crypt_read_crypt_data(), xdes_get_descriptor_const(): Pass the
BUF_GET_POSSIBLY_FREED flag to avoid the fatal error in buf_page_get_low()
if a DDL operation was just initiated.