diag: Fix possible usage of freed resource issue

Currently, there is a possibility of using already
freed memory while client reads from diag driver.
The patch adds proper protection to fix the issue.

CRs-Fixed: 2076623
Change-Id: Ic946865ac79f2684c06176be64fd2f3abc6048f7
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>