Return null if no charset matches

Fixes a segmentation fault if no charset matches. According to http://icu-project.org/apiref/icu4c/ucsdet_8h.html#aff2633b5055d472cff4108d94f97cf7d, ucsdet_detect() may return NULL if no charset matches.

Fixes: #15
Co-authored-by: chenzhip <letme2010@gmail.com>