gTrainers out-of-bounds data access

[AsparagusEduardo]

Description

As discussed on Discord, recently I found that tests and other situations that cause gTrainerBattleOpponent_A/B to be set to values equal or higher than TRAINERS_COUNT, making it so that it reads data out bounds. This manifested in #4022's tests after #4176 was merged, causing it to set up Electric Terrain at the beginning of the tests.

Version

upcoming (Edge)

Upcoming/master Version

5e79fcd

Discord contact info

AsparagusEduardo

[AsparagusEduardo]

#4216