Merge pull request dotnet#437 from bbowyersmyth/MemoryMapped_IntPtr

Change SafeMemoryMappedFileHandle to use IntPtr.Zero instead of new IntPtr(0)

src/System.IO.MemoryMappedFiles/src/Microsoft/Win32/SafeMemoryMappedFileHandle.cs

@@ -32,7 +32,7 @@ public override bool IsInvalid
             [SecurityCritical]
             get
             {
-                return handle == new IntPtr(0) || handle == new IntPtr(-1);
+                return handle == IntPtr.Zero || handle == new IntPtr(-1);
             }
         }
     }