Bluetooth: Controller: Fix incorrect auto variable init

Coverity analysis discovered NULL pointer being
dereferenced when passing a auto variable. The variable is
now correctly assigned with address of a valid default
value variable. As per design, the dereferencing will not
happen as the master role does not use the passed parameter
only slave role uses it to prepare the connection parameter
request PDU.

Change-id: I3f8519b23a83cb8c50c7fba81810eff7737ff74a
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>