Issue Description:

I haven't been able to pinpoint the problem exactly so far but in some cases Data.replaceSubrange can lead to an empty Data object.

var data = Data(bytes: [0x01, 0x02])

var dataI = data
dataI.replaceSubrange(0..<1, with: Data()) // Works as expected: [0x02]

var dataII = Data(base64Encoded: data.base64EncodedString())!
dataII.replaceSubrange(0..<1, with: Data()) // BUG: result is empty, 0 bytes

var dataIII = Data(base64Encoded: data.base64EncodedString())!
dataIII.replaceSubrange(0..<1, with: []) // Works again: [0x02]

In the code above, replacing the first byte with empty Data (aka removing it) only works in case I and III. The code only varies in how the modified Data object and the replacing data are constructed.

In our production code the original data object got constructed by using the empty initializer and by appending data constructed with init(bytes: UnsafeRawPointer, count: Int) by calling append(_ other: Data). So there seems to be some variation in how this bug is triggered.

The code above (case II) worked with Swift 3.0 / Xcode 8.2.1. We worked around the problem now by using the code in case III above.