mailbox: Off by one in mbox_test_message_read()

We need to leave space for the NUL char.

Fixes: 8ea4484d0c2b ('mailbox: Add generic mechanism for testing Mailbox Controllers')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

drivers/mailbox/mailbox-test.c

@@ -143,7 +143,7 @@ static ssize_t mbox_test_message_read(struct file *filp, char __user *userbuf,
 	int l = 0;
 	int ret;
 
-	touser = kzalloc(MBOX_HEXDUMP_MAX_LEN, GFP_KERNEL);
+	touser = kzalloc(MBOX_HEXDUMP_MAX_LEN + 1, GFP_KERNEL);
 	if (!touser)
 		return -ENOMEM;