diag: Validate copying length against source buffer length

There a possibility of out-of-bound read because of not
validating source buffer length against length that about
to be copied. The patch adds proper check for validating
length before copying data

CRs-Fixed: 2163793
Change-Id: I7c93839d0c4d83024ce23a0ce494d09dd08567a9
Signed-off-by: Hardik Arya <harya@codeaurora.org>