Return if sBluetoothHfpInterface pointer is NULL

Usecase:
During HFP initialisation.

Failure:
NULL pointer dereferencing, might cause crash

Rootcause:
Pointer 'sBluetoothHfpInterface' checked for NULL
and is dereferenced later.

Fix:
Check if the sBluetoothHfpInterface pointer is NULL
and return from initializeNative API before
dereferencing the pointer.

Fixes: 110066199
Change-Id: I99a159d9a2062b64689b6b0f694732d08fb7bd44