EAP peer: Add check before calling getSessionId method

We should not call getSessionID method if it's not provided. This fixes
a regression from commit 950c5630768285e460aa336c78a18162fb3e0434 where
EAP methods that did not implement getSessionId resulted in NULL pointer
dereference when deriving the key.

Change-Id: Iff5a34c2251e1e3bd8d014a31465e187a4817551
Signed-off-by: Shijie Zhang <shijiez@qca.qualcomm.com>