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Issue Description:

A `Data` can be constructed empty: The expression `let data = Data()` is valid and does not crash.

Supporting an empty `Data` is useful. It allows for consistent and generic code that doesn't need to consider additional details when a source of data has become exhausted, or when there is no data to work with.

It seems reasonable to expect that the method `advanced(by🙂` on `Data` should be able to handle a case where the resulting `Data` will be empty.

However, these examples crash:

```

let data1 = Data().advanced(by:0)

let data2 = Data([UInt8()]).advanced(by:1)

```

The error is:

"error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0). The process has been left at the point where it was interrupted, use "thread return -x" to return to the state before expression evaluation."

I've attached a playground screen shot that shows more detail about the crash being in `Foundation.Data.advanced(by🙂`.

This happens in Playgrounds and Xcode unit tests. I've not tested other platforms.