Fixes crash when DispatchData is created from an UnsafeBufferPointer<Uint8> with a nil address.
Fixes crash when DispatchData is created from an UnsafeBufferPointer<Uint8> with a nil address.

Radar 29337927

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch