ntdll: Preserve untouched parts of xstate in set_thread_wow64_context().

dlls/ntdll/unix/signal_x86_64.c

@@ -1281,9 +1281,11 @@ NTSTATUS set_thread_wow64_context( HANDLE handle, const void *ctx, ULONG size )
     {
         CONTEXT_EX *context_ex = (CONTEXT_EX *)(context + 1);
         XSAVE_AREA_HEADER *xs = (XSAVE_AREA_HEADER *)((char *)context_ex + context_ex->XState.Offset);
+        UINT64 mask = frame->xstate.Mask;
 
         if (xstate_compaction_enabled) frame->xstate.CompactionMask |= xstate_extended_features();
         copy_xstate( &frame->xstate, xs, xs->Mask );
+        if (xs->CompactionMask) frame->xstate.Mask |= mask & ~xs->CompactionMask;
         frame->restore_flags |= CONTEXT_XSTATE;
     }
     return STATUS_SUCCESS;