gconv: Correct Big5-HKSCS conversion to preserve all state bits. [BZ #25744]

This patch corrects the Big5-HKSCS converter to preserve the lowest 3 bits of
the mbstate_t __count data member when the converter encounters an incomplete
multibyte character.

This fixes BZ #25744.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>