com_ff(): Avoid "unsigned i".

com_ff(): Avoid "unsigned i".

It's an act of violence. The change should have no visible effect in
the common cases

	ff n and ff n-

where n is a (small) non-negative integer. The behaviour of the command
changes for negative values of n, but this case was never documented.

After this patch negative arguments instruct the subcommand to jump
backwards, which should meet the expected behaviour.