[pygobject] Revert "Revert "pygi-boxed: make in-place copy safer""

[Christoph Reiter <creiter src gnome org>]

commit 26b7fc3fee55870cc944b0f718da8a75c75f4eae
Author: Mathieu Duponchelle <mathieu centricular com>
Date:   Fri Feb 16 00:45:40 2018 +0100

    Revert "Revert "pygi-boxed: make in-place copy safer""
    
    This reverts commit 700c785367975995d24a8ffd00b2aa028ccf77e8.

 gi/pygi-boxed.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)
---
diff --git a/gi/pygi-boxed.c b/gi/pygi-boxed.c
index fa690bb3..599d6d3e 100644
--- a/gi/pygi-boxed.c
+++ b/gi/pygi-boxed.c
@@ -42,6 +42,8 @@ boxed_del (PyGIBoxed *self)
     if ( ( (PyGBoxed *) self)->free_on_dealloc && boxed != NULL) {
         if (self->slice_allocated) {
             g_slice_free1 (self->size, boxed);
+            self->slice_allocated = FALSE;
+            self->size = 0;
         } else {
             g_type = pyg_type_from_object ( (PyObject *) self);
             g_boxed_free (g_type, boxed);
@@ -203,7 +205,11 @@ void
 _pygi_boxed_copy_in_place (PyGIBoxed *self)
 {
     PyGBoxed *pygboxed = (PyGBoxed *)self;
-    gpointer copy = g_boxed_copy (pygboxed->gtype, pyg_boxed_get_ptr (self));
+    gpointer ptr = pyg_boxed_get_ptr (self);
+    gpointer copy = NULL;
+
+    if (ptr)
+        copy = g_boxed_copy (pygboxed->gtype, ptr);
 
     boxed_del (self);
     pyg_boxed_set_ptr (pygboxed, copy);