Assign nullopt to remove unnecessary log

Once the client requests idmap2d to release a directory iterator,
idmap2d should follow the client request to free and release
frro_iter_.

It should not only check the value of frro_iter_ but also make
frro_iter_ to be nullopt in releaseFabricatedOverlayIterator.

Fixes: 244389702
Test: adb reboot; adb wait-for-device; adb logcat -s idmap2d
Change-Id: Ida1e7e7ae89f12407d26e66ab318a91e4f861df4