Improve status check of LFUN_BUFFER_ZOOM_{OUT,IN}

Improve status check of LFUN_BUFFER_ZOOM_{OUT,IN}

buffer-zoom-out can actually zoom in (by giving a positive number as
the argument), so it should only be disabled at the minimum zoom
level if it is indeed zooming out.

Similarly, buffer-zoom-in can zoom out, so it should be disabled if
the user is at the minimum zoom and it is called to zoom out.

This commit amends 8884c404.