If no keyspace is specified when taking snapshot, there will be a segfault
because keynames is unconditionally dereferenced. Let's return an error
because a keyspace must be specified when column families are specified.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <20200427195634.99940-1-raphaelsc@scylladb.com>