tools/nolibc/printf: Move snprintf length check to callback

tools/nolibc/printf: Move snprintf length check to callback

Move output truncation to the snprintf() callback.
This simplifies the main code and fixes truncation of padded fields.

Add a zero length callback to 'finalise' the buffer rather than
doing it in snprintf() itself.

Fixes: e90ce42e81381 ("tools/nolibc: implement width padding in printf()")
Signed-off-by: David Laight <david.laight.linux@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260302101815.3043-3-david.laight.linux@gmail.com
[Thomas: clean up Fixes trailer]
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>