Adler32 isn't initialized correctly

[ehrmann]

The code that calls out to the adler32 implementation uses an initial value of 0, not 1, so it won't be compatible with other adler32 implementations (though there are workarounds).

The easiest way to verify is computing an empty checksum:

printf("Empty adler32: %llu\n", ComputeAdler32("", 0));

Compare this to Java or Python's implementations. Or the spec (see "Appendix: Sample code").

Fixing this with a breaking change is probably more trouble than it's worth; there's enough deltas out there that it would cause people problems, and this shouldn't(?) affect the quality of the checksum since 0 is a valid adler32 value, but it'd be a good idea to document it somewhere.