Fix invisible group mobs bug

This is a simplified adaptation of
29e7bd2 from Retrobution.

Co-authored-by: Jade <[email protected]>

Author: dongresource

src/MobAI.cpp

@@ -98,11 +98,17 @@ void MobAI::groupRetreat(Mob *mob) {
         }
         Mob* followerMob = (Mob*)NPCManager::NPCs[leadMob->groupMember[i]];
 
+        if (followerMob->state != MobState::COMBAT)
+            continue;
+
         followerMob->target = nullptr;
         followerMob->state = MobState::RETREAT;
         clearDebuff(followerMob);
     }
 
+    if (leadMob->state != MobState::COMBAT)
+        return;
+
     leadMob->target = nullptr;
     leadMob->state = MobState::RETREAT;
     clearDebuff(leadMob);