Assertion failures for workers that can't continue irrigating after GW

[XHawk87]

Describe the bug
After climate change triggers, we intermittently get assertion failures in the logs for workers attempting to do work in adjacent tiles.

To Reproduce
Steps to reproduce the behaviour:

1. Start up Chaos with Inaccessible Climates active.
2. Play turns until GW triggers, and occasionally we see assertion failures.

Expected behaviour
AI workers continue working. No errors in the logs

Screenshots

Platform and version (please complete the following information):

• OS: Linux
• Freeciv21 version: 3.1-rc.1
• Ruleset/Longturn game (if applicable): Chaos

Additional context

[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: Assertion false failed
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: ( 125, 171) Tribal Workers has activity Irrigate, but it can't continue at Plains
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: Please report this message at https://github.com/longturn/freeciv21/issues
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: Assertion false failed
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: ( 125, 171) Tribal Workers has activity Irrigate, but it can't continue at Plains
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: Please report this message at https://github.com/longturn/freeciv21/issues
Game saved as freeciv-T0021-Y-2000-auto.sav
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: Assertion false failed
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: ( 125, 171) Tribal Workers has activity Irrigate, but it can't continue at Plains
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T021: Please report this message at https://github.com/longturn/freeciv21/issues
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T022: Assertion false failed
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T022: ( 125, 171) Tribal Workers has activity Irrigate, but it can't continue at Plains
[critical] freeciv21-server (/home/xhawk87/sources/freeciv21/server/sanitycheck.cpp:459) - T022: Please report this message at https://github.com/longturn/freeciv21/issues

freeciv-T0021-Y-2000-auto.zip

[lmoureaux]

Duplicate of #1064