An input BIG5-HKSCS character may be converted into at most 2 wchar_t
characters. After outputting the second whcar_t character (which was
saved in the converter state) we must reset the state. If we fail
to reset the state we will be stuck continually copying that
character to the output even if we have further input to consider.
We add a new test case that covers the 4 BIG5-HKSCS characters
that may become 2 wchar_t characters.
Reviewed-by: Tom Honermann <tom@honermann.net>
