progress: don't dereference before checking for NULL

In `stop_progress()`, we're careful to check that `p_progress` is non-NULL before we dereference it, but by then we have already dereferenced it when calling `finish_if_sparse(*p_progress)`. And, for what it's worth, we'll go on to blindly dereference it again inside `stop_progress_msg()`. We could return early if we get a NULL-pointer, but let's go one step further and BUG instead. The progress API handles NULL just fine, but that's the NULL-ness of `*p_progress`, e.g., when running with `--no-progress`. If `p_progress` is NULL, chances are that's a mistake. For symmetry, let's do the same check in `stop_progress_msg()`, too. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>