(ReviewsPeer): add foolproof check for leitnerbox 0 or NULL
(ReviewsPeer): add foolproof check for leitnerbox 0 or NULL

Prevent updating flashcard in the DB if a refactoring / bug causes
leitnerbox to be NULL or zero for whatever reason.

kanji-koohii/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii,kanji-koohii/kanji-koohii,fabd/kanji-koohii,kanji-koohii/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii,kanji-koohii/kanji-koohii,kanji-koohii/kanji-koohii