position get_gap_vector should always ends a sentinel record like {id, -1}

[bguo068]

Adding a sentinel record is good for providing information about the end position of the last effective record. This is important when the last position is far away from the end of the chromosome, in which case the label is ambiguous for the range to the right of the last position.

https://github.com/gbinux/ibdtools/blob/cd8c10da7effa5d7d8c48309dcd956f1b6557fe2/include/positions.hpp#L99-L149

[bguo068]

one more issue size_t prev_label with for loop would be overwritten. It should be outside of the loop