nvs: Fix deletion of the last entry added

Make sure that the last entry added is deleted correctly by storing the
fact that one was found in a local variable.

Fix by Laczen JMS <laczenjms@gmail.com>

Fixes #18813.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>