disp: msm: add length check for debugfs_ctrl

Add check for length of debugfs_ctrl value before
copying to user buffer invoked during read operation.

Change-Id: I199110992921d0ae3791129fa0bf1e51dcca9107
Signed-off-by: Prashant Singh <prasin@codeaurora.org>

msm/sde_dbg.c

@@ -3675,6 +3675,9 @@ static ssize_t sde_dbg_ctrl_read(struct file *file, char __user *buff,
 	pr_debug("%s: ctrl:0x%x len:0x%zx\n",
 		__func__, sde_dbg_base.debugfs_ctrl, len);
 
+	if (len < 0 || len >= sizeof(buf))
+		return 0;
+
 	if ((count < sizeof(buf)) || copy_to_user(buff, buf, len)) {
 		pr_err("error copying the buffer! count:0x%zx\n", count);
 		return -EFAULT;