transport/controller: Do not leave _server uninitialized
transport/controller: Do not leave _server uninitialized

If an exception happens after sharded<cql_server>.start() the
controller's _server pointer is left pointing to stopped sharded
server. This makes it impossible to start the server again (via
API) since the check for if (_server) will always be true.

This is the continuation of the ae4d5a60 fix.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla