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: 51bd6f291583 ("Add support for IPMB driver")
Signed-off-by: Colin Ian King <[e-mail]>
Message-Id: <[e-mail]>
Reviewed-by: Asmaa Mnebhi <[e-mail]>
Signed-off-by: Corey Minyard <[e-mail]>

drivers/char/ipmi/ipmb_dev_int.c

···
 		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;