ffi/pic.lua: fix memory leak with some unsupported PNG files
ffi/pic.lua: fix memory leak with some unsupported PNG files

koreader/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,Frenzie/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,koreader/koreader-base