Fixed marshaling of null UTF-32 wide strings.

Closes issue #606. Thanks to magnet31 for reporting the issue.

src/Runtime/Helpers.cs

@@ -9,6 +9,9 @@ namespace CppSharp.Runtime
     {
         public static string MarshalEncodedString(IntPtr ptr, Encoding encoding)
         {
+            if (ptr == IntPtr.Zero)
+                return null;
+
             var size = 0;
             while (Marshal.ReadInt32(ptr, size) != 0)
                 size += sizeof(int);

tests/Common/Common.Tests.cs

@@ -406,6 +406,7 @@ public class CommonTests : GeneratorTestFixture
         var ws = new TestWideStrings();
         var s = ws.WidePointer;
         Assert.That(ws.WidePointer, Is.EqualTo("Hello"));
+        Assert.That(ws.WideNullPointer, Is.EqualTo(null));
     }
 
     [Test]

tests/Common/Common.h

@@ -597,9 +597,11 @@ typedef const wchar_t * LPCWSTR;
 struct DLL_API TestWideStrings
 {
     LPCWSTR GetWidePointer();
+    LPCWSTR GetWideNullPointer();
 };
 
 LPCWSTR TestWideStrings::GetWidePointer() { return L"Hello"; }
+LPCWSTR TestWideStrings::GetWideNullPointer() { return 0; }
 
 enum struct MyEnum { A, B, C };