nvs: hardening against bad sector close

A badly written sector close ate that has correct crc8 could allow
jumps outside the assigned flash area. This behavior is fixed.

A possibility existed that a badly erased sector or a incomplete write
of a large item created a empty closed sector. This has been fixed by:
a. Erase verification.
b. Clearing such a sector at startup.

Fixes #34382

Signed-off-by: Laczen JMS <laczenjms@gmail.com>

Co-Authered-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>