Fix nullptr dereference in arakoon_cluster_connect_master
Fix nullptr dereference in arakoon_cluster_connect_master

_arakoon_cluster_node_who_master() might return successfully with
master being a NULL pointer (indicating "None"), so better check
for that before any dereferencing of this pointer.

Incubaid/crakoon,Incubaid/crakoon,Incubaid/crakoon,Incubaid/crakoon