Crash when zombie tries to spawn second reinforcement

Summary
The game crashes when a zombie tries to spawn a zombie reinforcement for the second time. (Relates to MC-272321)

Steps to Reproduce

1. Set Difficulty to Hard.

2. Either: Spawn a leader Zombie. (A zombie have 5% chance of being a leader zombie, which is capable of summoning reinforcements.)
   Or, spawn a zombie and run the following command to make it a leader:

   /attribute @e[type=zombie,limit=1] minecraft:zombie.spawn_reinforcements base set 1

3. Switch gamemode to Survival.

4. Attack the zombie using snowballs. (Other projectiles can also be used.)

5. One reinforcement zombie spawns.

6. Continue hitting the original zombie with snowballs.

Expected Result
Game does not crash.

Actual Result
Game crashes.