StayInLane(StayInLaneMode.Forward/Backward) when node.m_segment0 == 0

[kianzarrin]

The following code assumes that node.m_segment0 always exits:
(from https://github.com/krzychu124/Cities-Skylines-Traffic-Manager-President-Edition)

https://github.com/krzychu124/Cities-Skylines-Traffic-Manager-President-Edition/blob/ab806f57362a2db7decffdbb336da5b64b5cdc74/TLM/TLM/UI/SubTools/LaneConnectorTool.cs#L372
if ((sourceLaneMarker.SegmentIndex == 0) ^ (stayInLaneMode == StayInLaneMode.Backward)) {
    continue;
}

Therefore when node.m_segment0 == 0 and the user tries to stay in lane only in one direction it will not work.

Reproduce:

1. create a segment
2. connect to more segments to it in a T shaped junction.
3. bulldoze the segment from step 1
4. use the lane connector tool to select the node and press SHIFT+S multiple times.

Suggested fix:
Find the smallest i where node.GetSegment(i) ~= 0
replace sourceLaneMarker.SegmentIndex == 0 with sourceLaneMarker.SegmentIndex == i