Improve iconvbufs() to do more buffer size checks.

Improve iconvbufs() to do more buffer size checks.

- If iconv() returns EINVAL or EILSEQ and the error is being ignored, make
  sure that there is room in the output buffer to store the erroneous char.
- When accepting an erroneous char, be sure to break if there are no more
  input characters (without calling iconv() with a zero input length).