Ticket #47391 - deleting and adding userpassword fails to update the password
Bug description: Deleting and adding password operation causes
the crash since once an entry is written to and retrieved from
DB, it does not have unhashed#user#password in the entry any
more. The delete userpassword internally invokes delete unhashed
existed in the entry.
Fix description: Once the internal attribute value pair is
stored in an entry extension, the mod for the attribute is
changed to the "disabled" mode. Also, this patch adds the
stricter check for the NULL reference.
Reviewed by Nathan (Thank you!!)
(cherry picked from commit 4305deeaa41ea8703ebb9ea6529790dd609b3060)