purge may crash in lazy queue (3.6.0)

[lishuai87]

When only publisher alive (no consume history), purge won't call purge_betas_and_deltas as Q3 is empty. This cause vqstate record wrong delta, len...
then consume will fail with 541 error: INTERNAL_ERROR

rabbit.log.txt
rabbit-sasl.log.txt

** Reason for termination == 
** {{{function_clause,
         [{rabbit_variable_queue,d,
              [{delta,16384,100000,100000}],
              [{file,"src/rabbit_variable_queue.erl"},{line,1100}]},
          {rabbit_variable_queue,maybe_deltas_to_betas,2,
              [{file,"src/rabbit_variable_queue.erl"},{line,2463}]},
          {rabbit_variable_queue,fetch_from_q3,1,
              [{file,"src/rabbit_variable_queue.erl"},{line,2418}]},
          {rabbit_variable_queue,queue_out,1,
              [{file,"src/rabbit_variable_queue.erl"},{line,1386}]},
          {rabbit_variable_queue,fetch,2,
              [{file,"src/rabbit_variable_queue.erl"},{line,615}]},
          {rabbit_priority_queue,fetch,2,
              [{file,"src/rabbit_priority_queue.erl"},{line,298}]},
          {rabbit_amqqueue_process,fetch,2,
              [{file,"src/rabbit_amqqueue_process.erl"},{line,643}]},
          {rabbit_queue_consumers,deliver_to_consumer,4,
              [{file,"src/rabbit_queue_consumers.erl"},{line,236}]}]},
     {gen_server2,call,
         [<0.255.0>,
          {basic_consume,false,<0.1974.0>,<0.1973.0>,false,0,
              <<"amq.ctag-ATvBjzQzVEqYu6qwWdy3cg">>,false,[],
              {'basic.consume_ok',<<"amq.ctag-ATvBjzQzVEqYu6qwWdy3cg">>}},
          infinity]}},
    [{gen_server2,call,3,[{file,"src/gen_server2.erl"},{line,340}]},
     {rabbit_channel,'-basic_consume/8-fun-0-',8,
         [{file,"src/rabbit_channel.erl"},{line,1499}]},
     {rabbit_misc,with_exit_handler,2,
         [{file,"src/rabbit_misc.erl"},{line,486}]},
     {rabbit_channel,basic_consume,8,
         [{file,"src/rabbit_channel.erl"},{line,1496}]},
     {rabbit_channel,handle_method,3,
         [{file,"src/rabbit_channel.erl"},{line,1078}]},
     {rabbit_channel,handle_cast,2,
         [{file,"src/rabbit_channel.erl"},{line,454}]},
     {gen_server2,handle_msg,2,[{file,"src/gen_server2.erl"},{line,1049}]},
     {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,240}]}]}