zero-pad the hash-string to 16 chars

I noticed that subberthehut would not find hash-matches for some
videos, while the XBMC OpenSubtitles.org-Plugin would.

wireshark revealed that XBMC was zero-padding the hash-value
to 16 characters which subberthehut didn't do. Meaning that subberthehut
would fail to find hash-matches for any hash-value that started with
four zero bits.

Fix this by letting asprintf() zero-pad the string to 16 characters.