Correct Data slice iteration to be indexed by the startIndex to the endIndex

Foundation/Data.swift

@@ -1126,7 +1126,7 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
     @inline(__always)
     private func _copyBytesHelper(to pointer: UnsafeMutableRawPointer, from range: NSRange) {
         if range.length == 0 { return }
-        memcpy(UnsafeMutableRawPointer(pointer), _backing.bytes!.advanced(by: range.location + _sliceRange.lowerBound), range.length)
+        memcpy(UnsafeMutableRawPointer(pointer), _backing.bytes!.advanced(by: range.location), range.length)
     }
 
     /// Copy a subset of the contents of the data to a pointer.

TestFoundation/TestNSData.swift

@@ -96,6 +96,7 @@ class TestNSData: XCTestCase {
             ("test_sliceAppending", test_sliceAppending),
             ("test_replaceSubrange", test_replaceSubrange),
             ("test_sliceWithUnsafeBytes", test_sliceWithUnsafeBytes),
+            ("test_sliceIteration", test_sliceIteration),
         ]
     }
 
@@ -1091,5 +1092,16 @@ extension TestNSData {
         }
         XCTAssertEqual(segment, [UInt8(2), UInt8(3)])
     }
+
+    func test_sliceIteration() {
+        let base = Data([0, 1, 2, 3, 4, 5])
+        let slice = base[2..<4]
+        var found = [UInt8]()
+        for byte in slice {
+            found.append(byte)
+        }
+        XCTAssertEqual(found[0], 2)
+        XCTAssertEqual(found[1], 3)
+    }
 }