fix win32 error

Could not run it on Win32, I modified it, it would be nice.

    Why does leveldb_free not work on your system?
    becase I build leveldb with msvc, so free(in c files) and C.free(in go) is diff implemention.

    Why is the db.Close in leveldb_test important?
    Will failed to Delete test temp directory after unit test is end.
    Must close file handle before delete the file on the win32 platform.