Some bugs found while fuzzing

[killercup]

I've been giving libfuzzer a try and added a target for serde-yaml (see rust-fuzz/targets#46).

After a few million runs, it found a few things: https://gist.github.com/killercup/c6c2d4bddd9bb9eadacff3324828fc28

• thread '<unnamed>' panicked at 'Out of bounds access'
• stack overflows

(Sadly, I forgot to enable RUST_BACKTRACE…)

[killercup]

Just saw fuzzing works on macOS now as well, and while testing I found a stack overflow ("AddressSanitizer: stack-overflow on address"): https://gist.github.com/killercup/6c20ed12f659989587f2dffc2193cfa0. This may be a bug in ASAN on mac, though, I haven't investigated any further.

[killercup]

Similar, but different stack overflow: https://gist.github.com/killercup/de51bc2cdf2a3a1b82f0e4ef650aa74d

[dtolnay]

Thanks! All of these have been fixed as of 0.8.4.