i18n: Only close valid iconv converters in i18n_done().

iconv_open() returns (iconv_t) -1 to indicate an error.  We shouldn't
pass this to iconv_Close().

Reported-by: Jeremy Lavergne <jeremy@lavergne.gotdns.org>.
John Darrington suggested that this was probably the problem, and
Jeremy confirmed it with valgrind.