Write custom header up to and including size, not excluding.

dlls/ole32/marshal.c

diff --git a/dlls/ole32/marshal.c b/dlls/ole32/marshal.c
index 1fe0340..0db2bec 100644
--- a/dlls/ole32/marshal.c
+++ b/dlls/ole32/marshal.c
@@ -1351,7 +1351,7 @@
 
     if (objref.flags & OBJREF_CUSTOM)
     {
-        ULONG custom_header_size = FIELD_OFFSET(OBJREF, u_objref.u_custom.size) - 
+        ULONG custom_header_size = FIELD_OFFSET(OBJREF, u_objref.u_custom.pData) - 
                                    FIELD_OFFSET(OBJREF, u_objref.u_custom);
         HGLOBAL hGlobal;
         LPVOID data;