When computing line-art, don't ref the bucket-fill tool in the
async data, and rather cancel any ongoing async upon tool
destruction, so that the async callback doesn't attept to touch the
now-dead tool.  This avoids segfaulting in the async callback when
switching to a different tool, while a line-art async operation is
active.

Additionally, always cancel any previous async operation in
gimp_bucket_fill_compute_line_art(), even if not starting a new
one.