diag: Fix possible underflow/overflow issues

Add check in order to fix possible integer underflow
during HDLC encoding which may lead to buffer
overflow. Also added check for packet length to
avoid buffer overflow.

Bug: 28767796
Change-Id: I198ef82c72687732e46893013dc21ef3b85244e2
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>
Signed-off-by: Yuan Lin <yualin@google.com>