Previously, each progress timer function registered with
start_update_progress() was directly used as the timer function of
progress.timer, so it was responsible of erasing the expired timer ID
from that member. Failing to do this could result in heap corruption.
The progress timer functions normally fulfilled the requirement by
calling update_progress(), but one such function upload_stat_timer()
had to erase the timer ID on its own too.
Now instead, there is a wrapper function progress_timeout(), which
progress.c sets as the timer function of progress.timer. This wrapper
erases the expired timer ID from progress.timer and then calls the
progress timer function registered with start_update_progress(). So
the progress timer function is no longer responsible of erasing the
timer ID and there's no risk that it could fail to do that in some
error situation.
This commit introduces a new risk though. Previously, if the struct
progress was freed while the timer was running, the (progress) timer
function would still be called, and it would be able to detect that
the progress pointer is NULL and recover from this situation. Now,
the timer function progress_timeout() has a pointer to the struct
progress and will dereference that pointer without being able to check
whether the structure has been freed. Fortunately, done_progress()
asserts that the timer is not running, so this should not occur.
