Sometimes when setting an igtk key the station maybe NULL.
In the of case igtk the function will skip to the end, and
try to print sta->addr, if sta is Null - we will access a
Null pointer.

Avoid accessing a Null pointer when setting a igtk key &
the sta == NULL, and print a default MAC address instead.
Signed-off-by: Matti Gottlieb <matti.gottlieb@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>