handleClientsBlockedOnQueues(): check that queue->list exists
handleClientsBlockedOnQueues(): check that queue->list exists

If the last client disconnected before being processed, the queue will
still be in the list of ready queues, the queue may also exist, but
queue->list may just be NULL.

Related to issue #72 second stack trace.

c2j/disque,CodyReichert/disque,felixbuenemann/disque,ErikDubbelboer/disque,tempbottle/disque,hanicker/disque,mopuri-sandeep/disque,lamby/pkg-disque,jmptrader/disque,gideonairex/disque,themage/disque,tempbottle/disque,c2j/disque,CodyReichert/disque,jmptrader/disque,felixbuenemann/disque,antirez/disque,ErikDubbelboer/disque,themage/disque,hanicker/disque,jmptrader/disque,chen-tian/disque,gideonairex/disque,lamby/disque,themage/disque,Koed00/disque,GrimDerp/disque,chen-tian/disque,seppo0010/disque,lamby/pkg-disque,GrimDerp/disque,Koed00/disque,antirez/disque,seppo0010/disque,mopuri-sandeep/disque,lamby/disque,sunheehnus/disque,sunheehnus/disque