simple_list forced to reset itself

(gdb) bt
#0  0x00504402 in __kernel_vsyscall ()
#1  0x002e1b10 in raise () from /lib/libc.so.6
#2  0x002e3421 in abort () from /lib/libc.so.6
#3  0x0816b4d6 in core_dump_real (who=0x81b77f2 "lists.c", line=264) at utils.c:833
#4  0x0810916c in simple_list (pList=0xb446f68) at lists.c:264
#5  0x080e56d7 in perform_violence () at fight.c:1231
#6  0x080b54c5 in heartbeat (heart_pulse=56080) at comm.c:996
#7  0x080b8178 in game_loop (local_mother_desc=3) at comm.c:941
#8  0x080b9d37 in init_game (argc=Cannot access memory at address 0x391b
) at comm.c:536
#9  main (argc=Cannot access memory at address 0x391b
) at comm.c:356




/* control the fights going on.  Called every 2 seconds from comm.c. */
void perform_violence(void)
{
  struct char_data *ch, *tch;

  for (ch = combat_list; ch; ch = next_combat_list) {
    next_combat_list = ch->next_fighting;

    if (FIGHTING(ch) == NULL || IN_ROOM(ch) != IN_ROOM(FIGHTING(ch))) {
      stop_fighting(ch);
      continue;
    }

    if (IS_NPC(ch)) {
      if (GET_MOB_WAIT(ch) > 0) {
        GET_MOB_WAIT(ch) -= PULSE_VIOLENCE;
        continue;
      }
      GET_MOB_WAIT(ch) = 0;
      if (GET_POS(ch) < POS_FIGHTING) {
        GET_POS(ch) = POS_FIGHTING;
        act("$n scrambles to $s feet!", TRUE, ch, 0, 0, TO_ROOM);
      }
    if ((GET_HIT(FIGHTING(ch)) <= 12) && (MOB_FLAGGED(ch, MOB_MERCY))) {
	stop_fighting(FIGHTING(ch));
	stop_fighting(ch);
        act("$n says: Blessed are the merciful, for they shall obtain mercy!", FALSE, ch, 0, 0, TO_ROOM);
    }
      // re-wield weapon lost to SKILL_DISARM
      if (LOST_WEAPON(ch)) {
        if (DISARM_WAIT(ch) > 0) {
	DISARM_WAIT(ch) -= PULSE_VIOLENCE;
	} else {
	do_wield(ch, OBJN(LOST_WEAPON(ch), ch), 0, 0);
        LOST_WEAPON(ch) = NULL;
      }
    }
}

    if (GET_POS(ch) < POS_FIGHTING) {
      send_to_char(ch, "You can't fight while sitting!!\r\n");
      continue;
    }
    if (GET_MOVE(ch) <= 0)
      continue;

    if (GROUP(ch)) {
      while ((tch = (struct char_data *) simple_list(GROUP(ch)->members)) != NULL) {		<<<---Line 1231
        if (tch == ch)
          continue;
        if (!IS_NPC(tch) && !PRF_FLAGGED(tch, PRF_AUTOASSIST))
          continue;
        if (IN_ROOM(ch) != IN_ROOM(tch))
          continue;
        if (FIGHTING(tch))
          continue;
        if (GET_POS(tch) != POS_STANDING)
          continue;
        if (!CAN_SEE(tch, ch))
          continue;
      
        do_assist(tch, GET_NAME(ch), 0, 0);				  
      }
    }
    {
    hit(ch, FIGHTING(ch), TYPE_UNDEFINED);
    GET_MOVE(ch) -= 1;
    }