Fixed a error where logging callback could be set to NULL and added some explaining comments.
Fixed a error where logging callback could be set to NULL and added some explaining comments.

vsmontalvao/liblouis,hammera/liblouis,BueVest/liblouis,vsmontalvao/liblouis,IndexBraille/liblouis,BueVest/liblouis,IndexBraille/liblouis,BueVest/liblouis,hammera/liblouis,IndexBraille/liblouis,hammera/liblouis,vsmontalvao/liblouis,IndexBraille/liblouis,hammera/liblouis,vsmontalvao/liblouis,liblouis/liblouis,BueVest/liblouis,vsmontalvao/liblouis,liblouis/liblouis,BueVest/liblouis,IndexBraille/liblouis,liblouis/liblouis,hammera/liblouis,BueVest/liblouis,liblouis/liblouis,hammera/liblouis,liblouis/liblouis,liblouis/liblouis