Fix UB in DoRead::read_from()

[sdroege]

We need to initialize the buffer first before creating a byte slice from
it and passing it to Read::read().

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See also comment in the code. This kind of defeats the purpose of getting uninitialized data to begin with, and is probably a good argument for not having this kind of API in tokio::AsyncRead and bytes to begin with until there's API in std to handle this in a more useful way.

[daniel-abramov]

FYI @agalakhov

[sdroege]

There is discussion for improving the API in tokio 0.3 etc, but I think as long as we have to deal with a Read there's nothing that can be improved here.