Audit libflate

[Shnatsel]

https://crates.io/crates/libflate

DEFLATE compression/decompression, 3000 downloads/day, exposed to untrusted data, has unsafe code.

[Shnatsel]

I've done the audit. Issues discovered:
sile/libflate#29
sile/libflate#31
sile/libflate#33
sile/libflate#35

I've also fixed the code and dropped unsafe blocks where possible:
sile/libflate#30
sile/libflate#32
sile/libflate#34
sile/libflate#36
sile/libflate#37
sile/libflate#38

[Shnatsel]

Unsafe blocks that could not be removed without regressing performance are blocked by:

1. Reading a CString safely without overhead from Read rust-lang/rust#59229 for reading a CString from a stream without checking all bytes for zeroes twice
2. RFC for Vec::append_from_within() rust-lang/rfcs#2714 for efficiently appending contents of a vector to itself

[Shnatsel]

Security advisories:

https://rustsec.org/advisories/RUSTSEC-2019-0010.html

and we still need to figure out if sile/libflate#33 is exploitable or not. Once that's done this issue can be closed (and the blockers moved to some kind of central tracker, I guess markdown files in this repo?)

[Shnatsel]

Exploitability analysis done, the flaw is found to be not exploitable. We're done!

[mleonhard]

sile/libflate#67 adds forbid(unsafe_code) 🎉