Check for end iterator before using iterator

Check for end iterator before using iterator

If it == end, then sequence_length(it) will be called which will attempt to dereference the it iterator. This is normally harmless, because the get_sequence_x() functions each check to see whether it == end. However, some runtime libraries (MSVC CRT debug build in particular) check the validity of every iterator dereference, and a runtime check will be triggered inside sequence_length() if it is at the end.