diag: Add NULL pointer check for write buffer of fwd_info

Currently there is a possibility of NULL pointer dereference
because of missing NULL pointer check for writer buffer of
fwd_info. The patch adds NULL pointer check before accessing
write buffer buf_ptr of fwd_info.

CRs-Fixed: 2193526
Change-Id: Ie79eb1c7e3635210c09d4fbb1c751c6a908196ee
Signed-off-by: Hardik Arya <harya@codeaurora.org>