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 <aurelien@aurel32.net>