Correctly handle cases where detected data contains null-bytes

[jaant]

zbar's zbar_symbol_get_data() API function can return data that contains null-bytes. this patch adds support for such cases.

[coveralls]

Coverage remained the same at 98.095% when pulling 62401e8 on jaant:master into 833b375 on NaturalHistoryMuseum:master.

[micsthepick]

I have tested this code with a QR that does contain (many) null characters, and it appears to have worked well

[quicklizard99]

Thanks very much @jaant I would like to make some changes so will raise a fresh PR that includes your commits