Fix LengthDelimitedCodec buffer over-reservation.

[yotamofek]

The LengthDelimitedCodec calls BytesMut::reserve on two occasions:

• after the header is read, to allocate memory for the expected frame
• after a frame is read, to allocate memory for another header that may follow

In both cases, the implementation currently calls src.reserve(n) which reserves the necessary n bytes for additional data, disregarding the fact that the buffer might already have additional data present that just wasn't decoded yet.

Motivation

This causes excess memory allocation, which might be quite substantial in certain cases.

Solution

Take into account the buffer's current length when calculating the amount of memory that should be reserved.

As a small drive-by cleanup, I also removed a check that n != 0 before calling to src.advance(n), since that is redundant according to the docs:

A call with cnt == 0 should never panic and be a no-op.

Not sure if/how to write tests for this, any feedback would be welcome.

[Darksonn]

Thanks.