Fix memory leak with BB.fromstring()
Fix memory leak with BB.fromstring()

apletnev/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,houqp/koreader-base,NiLuJe/koreader-base,houqp/koreader-base,NiLuJe/koreader-base,apletnev/koreader-base,koreader/koreader-base,Frenzie/koreader-base,houqp/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,houqp/koreader-base,Frenzie/koreader-base,Frenzie/koreader-base,koreader/koreader-base,apletnev/koreader-base,apletnev/koreader-base,koreader/koreader-base