xkb: Escape non-printable characters correctly.

XkbStringText escapes non-printable characters using octal numbers. Such escape
sequence would be at most 5 characters long ("\0123"), so it reserves 5 bytes
in the buffer. Due to char->unsigned int conversion, it would print much longer
string for negative numbers.

Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Julien Cristau <jcristau@debian.org>
(cherry picked from commit eaf1f72e)