diag: Add protection while processing non-hdlc packets

Currently, there is possibility of out-of-bound accesses during
handling of data in non-hdlc path. The patch adds proper protection
when processing non-hdlc packet information to fix the issue.

CRs-Fixed: 2029216
Change-Id: I07c466f85bd8ac08226948fea86b1d8567e68431
Signed-off-by: Hardik Arya <harya@codeaurora.org>