Add SafeDCHandle default ctor

Fixes #427

Author: AArnott

src/User32/PublicAPI.Unshipped.txt

@@ -117,6 +117,7 @@ PInvoke.User32.MenuObject.OBJID_SYSMENU = 4294967295 -> PInvoke.User32.MenuObjec
 PInvoke.User32.SafeCursorHandle
 PInvoke.User32.SafeCursorHandle.SafeCursorHandle() -> void
 PInvoke.User32.SafeCursorHandle.SafeCursorHandle(System.IntPtr preexistingHandle, bool ownsHandle = true) -> void
+PInvoke.User32.SafeDCHandle.SafeDCHandle() -> void
 PInvoke.User32.SendMessageTimeoutFlags
 PInvoke.User32.SendMessageTimeoutFlags.SMTO_ABORTIFHUNG = 2 -> PInvoke.User32.SendMessageTimeoutFlags
 PInvoke.User32.SendMessageTimeoutFlags.SMTO_BLOCK = 1 -> PInvoke.User32.SendMessageTimeoutFlags

src/User32/User32+SafeDCHandle.cs

@@ -21,6 +21,14 @@ public class SafeDCHandle : SafeHandle
             /// </summary>
             public static readonly SafeDCHandle Null = new SafeDCHandle(IntPtr.Zero, IntPtr.Zero);
 
+            /// <summary>
+            /// Initializes a new instance of the <see cref="SafeDCHandle"/> class.
+            /// </summary>
+            public SafeDCHandle()
+                : base(IntPtr.Zero, true)
+            {
+            }
+
             /// <summary>
             /// Initializes a new instance of the <see cref="SafeDCHandle"/> class.
             /// </summary>