GVariant: clear memory before releasing it

Bug #640807 makes a reasonable case for why it's better to have your
program crash outright in the case of memory errors.  With this
modification, GVariant is far more likely to do that in the case that a
GVariant pointer is used shortly after being freed.

glib/gvariant-core.c

@@ -629,6 +629,7 @@ g_variant_unref (GVariant *value)
       else
         g_variant_release_children (value);
 
+      memset (value, 0, sizeof (GVariant));
       g_slice_free (GVariant, value);
     }
 }