[libdispatch-data-fixes] DispatchDataIterator crashes when iterating DispatchData.empty

    *  DispatchDataIterator is unsafe when iterating the empty
    DispatchData object as it forcibly unwraps a nil pointer.

Author: mwwa

stdlib/public/SDK/Dispatch/Data.swift

@@ -237,8 +237,9 @@ public struct DispatchDataIterator : IteratorProtocol, Sequence {
 		self._count = 0
 		self._data = __dispatch_data_create_map(
 			_data as __DispatchData, &ptr, &self._count)
-		self._ptr = UnsafePointer(ptr!)
+		self._ptr = UnsafePointer(ptr)
 		self._position = _data.startIndex
+		assert(self._ptr != nil || self._count == self._position)
 	}
 
 	/// Advance to the next element and return it, or `nil` if no next
@@ -253,7 +254,7 @@ public struct DispatchDataIterator : IteratorProtocol, Sequence {
 	}
 
 	internal let _data: __DispatchData
-	internal var _ptr: UnsafePointer<UInt8>
+	internal var _ptr: UnsafePointer<UInt8>!
 	internal var _count: Int
 	internal var _position: DispatchData.Index
 }

test/1_stdlib/Dispatch.swift

@@ -61,3 +61,10 @@ if #available(OSX 10.10, iOS 8.0, *) {
     block.cancel()
   }
 }
+
+DispatchAPI.test("dispatch_data_t enumeration") {
+	// Ensure we can iterate the empty iterator
+	for x in DispatchData.empty {
+		_ = 1
+	}
+}