net: Add NULL check to eventhandler

The eventhandler gets called with NULL buf, but it can't handle it. Add
a NULL check to prevent crashing.

Change-Id: Id16c2aa093a145e0442dfc61dd0b6e1b81b701f5
Signed-off-by: Jaakko Hannikainen <jaakko.hannikainen@intel.com>