wlan: check BcnNumIes against size of header instead of 0

Currently, for while loop BcnNumIes is checked against 0
which may cause OOB read for len = *(pBcnIes + 1).

Fix is to check BcnNumIes against size of header i.e 2 instead
of 0 to avoid 00B read.

Change-Id: Id167410da790e449d36853d8505142e1b218e9b8
CRs-Fixed: 2635666