wlan: Avoid OOB read in dot11f_unpack_assoc_response

Avoid OOB read in dot11f_unpack_assoc_response API. Modify
the check to nBuf-len > 1 to read another byte of pBufRemaining.
This ensures a read of at least 2 bytes since all IEs have
at least 2 bytes.

Change-Id: If8f43739091b1baa8a86751fa60b9af2a90a751a
CRs-Fixed: 3064612