Fix units idling after unable to garrison while there are rally points.

When unable to garrison and that is the first rally point, the rest of the rally point queue is not handled.
The check was introduced in r14146 / rP14146 because UnitAI didn't guard garrisoning properly.

Patch by: @Langbart
Differential revision: https://code.wildfiregames.com/D4954
Fixes #6746