Ticket #48987 - Heap use after free in dblayer_close_indexes
Description: Once an attribute info is deleted, its backpointer
dblayer_handle_ai_backpointer in the dblayer handle needs to be
set to NULL not to access the address again. We also need to set
this to null from within the dblayer_close_indexes because there
is no guarantee on the order that we free the handle or the
attrinfo.
https://fedorahosted.org/389/ticket/48987
Author: nhosoi, wibrown
Review: nhosoi (Thanks!)
(cherry picked from commit 99176404bfe76ee9fcf48b8b28750ec3979ec020)