Running fuzzing tar test with [zig std lib
fuzzing](https://github.com/squeek502/zig-std-lib-fuzzing) reached and
assert in tar implementation. Assert (in std lib) should not be
reachable by external input, so I'm fixing this to return error.
Itarator has `next` function, iterates over tar files. When using from
outside of module with `tar.` prefix makes more sense.
var iter = tar.iterator(reader, null);
while (try iter.next()) |file| {
...
}
