drivers: ipmi: fix off-by-one bounds check that leads to a out-of-bounds write

The end of buffer check is off-by-one since the check is against
an index that is pre-incremented before a store to buf[]. Fix this
adjusting the bounds check appropriately.

Addresses-Coverity: ("Out-of-bounds write")
Fixes: 51bd6f2 ("Add support for IPMB driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Message-Id: <20200114144031.358003-1-colin.king@canonical.com>
Reviewed-by: Asmaa Mnebhi <asmaa@mellanox.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

Author: Colin Ian King

drivers/char/ipmi/ipmb_dev_int.c

@@ -279,7 +279,7 @@ static int ipmb_slave_cb(struct i2c_client *client,
 		break;
 
 	case I2C_SLAVE_WRITE_RECEIVED:
-		if (ipmb_dev->msg_idx >= sizeof(struct ipmb_msg))
+		if (ipmb_dev->msg_idx >= sizeof(struct ipmb_msg) - 1)
 			break;
 
 		buf[++ipmb_dev->msg_idx] = *val;