Only call free() on something we allocated.

If we were already provided a struct _citrus_iconv (e.g. through
iconv_open_into()), we should not call free() in case io_init_context()
fails. Instead, call it on the pointer of the allocated object, which
will be NULL in case of iconv_open_into().
Make some tiny improvements to iconv_open().

- Remove an unneeded variable.
- Fix whitespace bugs.
- Fix typoes in comment.
- Improve string handling a bit. Don't handroll strstr() and don't
  terminate a strdup()'ed string. Instead, simply strndup() the part we
  need.