Suit and accessory retailer Men's Wearhouse today said second quarter profit fell to $7.8 million from $10.3 million in the same period a year ago.   For the second quarter ended 3 August 2002 net sales increased 3.8 per cent to $308.6 million from $297.2 million last time. US comparable store sales dropped 1.5 per cent while those in Canada increased 2.2 per cent over last year's quarter.