ByteString.IndexOf throws IndexOutOfRangeException after concating two ByteStrings

[marcpiechura]

After concating two ByteString's IndexOf throws the exception if the element didn't exist.

Example:

var b = ByteString.Create(new byte[] {1}) + ByteString.Create(new byte[] {2});

b.IndexOf(3);

[wmeints]

Changed things around a bit. I found a better way to fix the problem.
After looking around a bit more, I think that HasNext should return false when there are no more sub iterators left in the list.

@Silv3rcircl3 Can you check if this works better for you?

[marcpiechura]

Closed via #1887