cons_as_sexpr does not handle encoding errors of snprintf */

[rexim]

introduced in tsoding/nothing#377

[rexim]

<X33Grawl> snprintf will never return -1 (error)
<X33Grawl> since the encoding error happened only on widechar
<X33Grawl> just make sure your glibc is higher than 2.0.6 since before 2.0.6, snprintf return -1 on truncated writing