diag: Prevent out-of-bound access while processing mask commands

Mask commands are being processed without proper check of buffer
length, which can lead to out of bound access. The patch adds
proper check for buffer length.

Change-Id: I6eb7ad01fb86f0cd0fb32390cfedd518428a64ba
Signed-off-by: Hardik Arya <harya@codeaurora.org>