fix: fill first empty waypoint slot instead of counting filled waypoints

[wursterje]

Problem

When the page is opened with only the destination pre-filled via URL (?loc=&loc=<dest>), clicking the map overwrites the destination instead of filling the empty start slot.

The waypoint array looks like this:

Index	latLng
0	null
1	{...}

addWaypoint() counted the number of filled waypoints (= 1) and derived the target slot from that count. Since it assumed filled waypoints are always packed at the front, it computed slot 1 and overwrote the destination.

Fix

Instead of counting filled waypoints, scan the array for the first slot without a coordinate and fill that. If all slots are filled, fall back to replacing the last one (previous behaviour).

Use case

Sites that link to the router with a pre-filled destination (?loc=&loc=<institution>) so visitors can set their own start point
with a single map click. The old behaviour destroyed the destination on that click.

Fixes #479

[DennisOSRM]

Thanks so much for the contribution. Will start the review sometime this week.

[Copilot]

Pull request overview

This PR fixes map-click waypoint placement when the route is opened with an empty start and a pre-filled destination (e.g. ?loc=&loc=<dest>), by selecting the first empty waypoint slot instead of deriving the target slot from the count of filled waypoints.

Changes:

• Store lrmControl.getWaypoints() in a local waypoints variable for reuse.
• Replace the “count-filled-waypoints → compute index” logic with “find first empty slot → fill it”.
• Keep existing behavior when all slots are filled by replacing the last waypoint.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[DennisOSRM]

@wursterje Please have a look at the comments from the review bot, especially the second comment on using max(.) to make sure the index doesn't become negative. Otherwise, this is looking good.