Fix crashes in ParseHeaders

[buger]

This PR should fix panics inside HTTP protocol code #408, and this case is quite interesting.

I could not replicate bug manually and decided to try a fuzzing.

Fuzzing or fuzz testing is a software testing technique, often automated or semi-automated, that involves providing invalid, unexpected, or random data to the inputs of a computer program.
https://en.wikipedia.org/wiki/Fuzzing

The basic idea that it is that fuzzer is massively scaled random generator, which tries to run your function with all possible combinations. And Golang have a really nice one: https://github.com/dvyukov/go-fuzz

It's quite easy to use it, for testing proto library command was:

// Creates zip file with binary
go-fuzz-build github.com/buger/gor/proto

// Runs the fuzzer and in current directory creates 3 directories: corpus, crashers, suppressions
go-fuzz -workdir ./ -bin ./proto-fuzz.zip

After it finished, we need to look at crashers directory, which will contain file with panic trace, and actual input which caused the issue.

Take a look PR itself to understand what it does.