msxml3: Correctly handle VT_NULL in IXMLHTTPRequest::send.

dlls/msxml3/httprequest.c

@@ -736,11 +736,14 @@ static HRESULT BindStatusCallback_create(httprequest* This, BindStatusCallback *
             /* fall through */
         case VT_EMPTY:
         case VT_ERROR:
+        case VT_NULL:
             ptr = NULL;
             size = 0;
             break;
         }
 
+        if (size)
+        {
             bsc->body = GlobalAlloc(GMEM_FIXED, size);
             if (!bsc->body)
             {
@@ -756,6 +759,7 @@ static HRESULT BindStatusCallback_create(httprequest* This, BindStatusCallback *
             send_data = GlobalLock(bsc->body);
             memcpy(send_data, ptr, size);
             GlobalUnlock(bsc->body);
+        }
 
         if (V_VT(body) == VT_BSTR)
             heap_free(ptr);