Motivation:
We need to always return a real slice even when the requested length is 0. This is needed as otherwise we not correctly share the reference count and so may leak a buffer if the user call release() on the returned slice and expect it to decrement the reference count of the "parent" buffer.
Modifications:
- Always return a real slice
- Add unit test for the bug.
Result:
No more leak possible when a user requests a slice of length 0 of a SlicedByteBuf.
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