mailbox: mailbox-test: Prevent memory leak

If we set the Signal twice or more, without using it as part of a message,
memory will be re-allocated and the pointer over-written. Prevent this
potential leak by only allocating memory when there isn't any already.

Reported-by: Dan Carpenter <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/mailbox-test.c

···
 		return -EINVAL;
 	}
 
-	tdev->signal = kzalloc(MBOX_MAX_SIG_LEN, GFP_KERNEL);
-	if (!tdev->signal)
-		return -ENOMEM;
+	/* Only allocate memory if we need to */
+	if (!tdev->signal) {
+		tdev->signal = kzalloc(MBOX_MAX_SIG_LEN, GFP_KERNEL);
+		if (!tdev->signal)
+			return -ENOMEM;
+	}
 
 	if (copy_from_user(tdev->signal, userbuf, count)) {
 		kfree(tdev->signal);