When requesting memory via `reader_permit::request_memory()`, the
requested amount is added to `_requested_memory` member of the permit
impl. This is because multiple concurrent requests may be blocked and
waiting at the same time. When the requests are fulfilled, the entire
amount is consumed and individual requests track their requested amount
with `resource_units` to release later.
There is a corner-case related to this: if a reader permit is registered
as inactive while it is waiting for memory, its active requests are
killed with `std::bad_alloc`, but the `_requested_memory` fields is not
cleared. If the read survives because the killed requests were part of
a non-vital background read-ahead, a later memory request will also
include amount from the failed requests. This extra amount wil not be
released and hence will cause a resource leak when the permit is
destroyed.
Fix by detecting this corner case and clearing the `_requested_memory`
field. Modify the existing unit test for the scenario of a permit
waiting on memory being registered as inactive, to also cover this
corner case, reproducing the bug.
Fixes: #13539Closes#13679
c1e8e86637