subsys: fs/nvs: break if no end of sector is found
subsys: fs/nvs: break if no end of sector is found

In case a sector is not empty nor properly closed (ie it never contains
8 times 0xff nor 0x00), the _nvs_prev_ate will loop indefinitely and
will start adressing memory outside of the flash area.

Fix that by stopping the loop when the address matches the beginning of
the sector.

Signed-off-by: Aurelien Jarno <bceb368e7f2cb351af47298f32034f0587bbe4a6@aurel32.net>

kraj/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,kraj/zephyr,finikorg/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,kraj/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,kraj/zephyr,nashif/zephyr,ldts/zephyr