progress.c: use dereferenced "progress" variable, not "(*p_progress)"

progress.c: use dereferenced "progress" variable, not "(*p_progress)"

Since 98a13647408 (trace2: log progress time and throughput,
2020-05-12) stop_progress() dereferences a "struct progress **"
parameter in several places. Extract a dereferenced variable to reduce
clutter and make it clearer who needs to write to this parameter.

Now instead of using "*p_progress" several times in stop_progress() we
check it once for NULL and then use a dereferenced "progress" variable
thereafter. This uses the same pattern as the adjacent
stop_progress_msg() function, see ac900fddb7f (progress: don't
dereference before checking for NULL, 2020-08-10).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>