Avoid a bad free in the encoding handling logic

See: https://launchpad.net/ubuntu/+source/a2ps/+changelog

Index: b/lib/encoding.c

===================================================================

--- a/lib/encoding.c

+++ b/lib/encoding.c

@@ -538,7 +538,9 @@

{

/* Find if there is a substitute for that font */

res = pair_get (encoding->substitutes, font_name);

- if (!res)

+ if (res)

+ res = xstrdup (res);

+ else

/* No. Check if this font is supported */

if (font_exists (job, font_name))

/* Avoid returning sth alloca'd */