Changes

fix stat_item leak in hnb_persistent_free()

Add missing stat_item free in hnb_persistent_free().

We recently fixed a rate_ctr leak in
I14e050bfb91b993f194e3800eacdc0d10f2b1a4e, but missed the also leaking
stat_item.

Particularly relevant with upcoming patch
Ic819d7cbc03fb39e98c204b70d016c5170dc6307 -- testing that patch revealed
the leak.

Change-Id: I7326c53d595dce7b442eced89ff8f4b972bd2a82