Bluetooth: controller: fixing error re. all zero chmap in conn-ind

When a connect indication contains a channel map of all zeros, the
adv->conn is left NULL'ed after return and next connect attempt leads
to a crash.

This was identified via the Sweyntooth test suite executing the script
re. issue 6.14 towards an Oticon target.

Signed-off-by: Erik Brockhoff <erbr@oticon.com>