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: I6dfe890c9db521c9144f05155c50b289c83b5b8f
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>
Signed-off-by: Yuan Lin <yualin@google.com>