fsmonitor: fix khash memory leak in do_handle_client

fsmonitor: fix khash memory leak in do_handle_client

The `shown` kh_str_t was freed with kh_release_str() at a point in
the code only reachable in the non-trivial response path.  When the
client receives a trivial response, the code jumps to the `cleanup`
label, skipping the kh_release_str() call entirely and leaking the
hash table.

Fix this by initializing `shown` to NULL and moving the cleanup to the
`cleanup` label using kh_destroy_str(), which is safe to call on NULL.
This ensures the hash table is freed regardless of which code path is
taken.

Signed-off-by: Paul Tarjan <github@paulisageek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>