Don't apply dungeon item validation to town items

[StephenCWills]

This PR fixes up the logic in IsItemValid() based on the validation logic located in UnPackNetItem().

devilutionX/Source/pack.cpp

Lines 443 to 457 in a12b536

	uint16_t creationFlags = SDL_SwapLE16(packedItem.item.wCI);
	uint32_t dwBuff = SDL_SwapLE16(packedItem.item.dwBuff);
	if (idx != IDI_GOLD)
	ValidateField(creationFlags, IsCreationFlagComboValid(creationFlags));
	if ((creationFlags & CF_TOWN) != 0)
	ValidateField(creationFlags, IsTownItemValid(creationFlags, player));
	else if ((creationFlags & CF_USEFUL) == CF_UPER15)
	ValidateFields(creationFlags, dwBuff, IsUniqueMonsterItemValid(creationFlags, dwBuff));
	else if ((dwBuff & CF_HELLFIRE) != 0 && AllItemsList[idx].iMiscId == IMISC_BOOK)
	return RecreateHellfireSpellBook(player, packedItem.item, &item);
	else
	ValidateFields(creationFlags, dwBuff, IsDungeonItemValid(creationFlags, dwBuff));
	RecreateItem(player, packedItem.item, item);
	return true;

Prior to this fix, items bought from Wirt by a clvl 50 character would always be unusable, despite Net Validation indicating success.