Before this patch, name_width becomes negative or null for width values
less than 15 and name_width values greater than 25 (default: 50). This
leads to output random data.
This patch checks for minimal width and name_width values.
Signed-off-by: Olivier Marin <dkr@freesurf.fr> Signed-off-by: Junio C Hamano <gitster@pobox.com>