Handle when chunked stream ends prematurely

[rylev]

We should have some sort of logic when the chunked stream ends prematurely. Right now we just end the stream and leave it to later steps to (maybe?) catch this issue: https://github.com/http-rs/async-h1/pull/69/files#r392666474

[yoshuawuyts]

I suspect we have an out of bounds read issue with chunked encoding as well. This might inform what to do about error handling.

[joshtriplett]

I managed to hit this case with a stream that doesn't end prematurely: #195