_citrus_iconv_std_iconv_convert can return EILSEQ. This is turned on in
e2030ca246a777c50a7aacb3a35f88addded21cf, which set ci_discard_ilseq =
false. As a result, we should check EILSEQ in the return value or it
will incorrectly return -2.
Fixes: e2030ca246a7 ("libc: fix c*rtomb/mbrtoc*")