bug fix: allow folder names of length 64 in leveldb

The file internal/files/leveldb.go contains code to convert between folder+device+name and byte slices used as database keys.  This code did not work when the folder name was 64 bytes long.  If folder names of length 64 are allowed, something like the attached patch may be useful. (Folder names containing 0 bytes also don't work; I am not sure whether this is a problem.)