storage_service: Fix load_broadcaster in get_load_map
storage_service: Fix load_broadcaster in get_load_map

If get_load_map is called from the api while load_broadcaster is not
set yet, we dereference nullptr.

Fixes #763.
Message-Id: <0949bfd84d2caf2c0282fb96a1565491ca982a28@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla