Verbose mode reports 1844674407370955161600% compression

[benrg]

When trying to compress incompressible buffers (like a single zero byte), verbose output produces lines like

Original Size: 1, Deflate: 3, Compression: 1844674407370955161600.000000% Removed

I didn't look at the code but I imagine it's an unsigned-wraparound issue.

[slashnick]

#138 should fix this