cld80211-lib: add null check for pointer *nla_data

The code logic should be checking the return value of
function 'nla_nest_start'. And code which invokes function
'cld80211_msg_alloc' will dereference pointer '*nla_data'.
To aovid null pointer dereference, need to add null check
before use.

Change-Id: Iaac52b4615e5d9103477ed50afc9d5718671c66a
CRs-Fixed: 2664794