diag: Prevent crash due to buffer over run

DIAG provides packet request/response service. A PC tool
can send a request and receive response from an app on
phone. In case, app is not running, the phone should return
an error response, which is almost same length as request.
A buffer overrun can happen, if the request is too huge. Thus,
adding a check to prevent buffer overrun.

Change-Id: I0955f9bcb703274bd1286047102c8303e6691c84
CRs-Fixed: 323841
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>