ref-filter.c: fix a leak in get_head_description

ref-filter.c: fix a leak in get_head_description

In 2708ce62d2 (branch: sort detached HEAD based on a flag, 2021-01-07) a
call to wt_status_state_free_buffers, responsible of freeing the
resources that could be allocated in the local struct wt_status_state
state, was eliminated.

The call to wt_status_state_free_buffers was introduced in 962dd7ebc3
(wt-status: introduce wt_status_state_free_buffers(), 2020-09-27).  This
commit brings back that call in get_head_description.

Signed-off-by: Rubén Justo <redacted>
Reviewed-by: Martin Ågren <redacted>
Acked-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>