Tentative fix of #1083

[ricardopereira]

#1083

The eventHandler from ARTEventListener holds a weak reference and if that handler is released then the dispatch_after will receive a queue that is nil.

I forced that situation and the stack trace is similar to the issue:

Thread 13 Queue : io.ably.main (serial)
#0  0x00000001005375d5 in _dispatch_continuation_init_slow ()
#1  0x000000010054d77a in dispatch_after ()
#2  0x00000001003107a2 in artDispatchScheduled at /Users/ricardopereira/Repositories/Whitesmith/AblyMacDemo/Pods/Ably/Source/Private/ARTGCD.m:47
#3  0x0000000100309f99 in -[ARTEventListener startTimer] at /Users/ricardopereira/Repositories/Whitesmith/AblyMacDemo/Pods/Ably/Source/ARTEventEmitter.m:141
#4  0x0000000100342678 in -[ARTRealtimeInternal transition:withErrorInfo:] at /Users/ricardopereira/Repositories/Whitesmith/AblyMacDemo/Pods/Ably/Source/ARTRealtime.m:486
#5  0x0000000100342772 in -[ARTRealtimeInternal transitionToDisconnectedOrSuspendedWithError:] at /Users/ricardopereira/Repositories/Whitesmith/AblyMacDemo/Pods/Ably/Source/ARTRealtime.m:498
#6  0x0000000100344ca8 in __45-[ARTRealtimeInternal transitionSideEffects:]_block_invoke_2 at /Users/ricardopereira/Repositories/Whitesmith/AblyMacDemo/Pods/Ably/Source/ARTRealtime.m:578
#7  0x0000000100324f03 in __38-[ARTOSReachability internalCallback:]_block_invoke at /Users/ricardopereira/Repositories/Whitesmith/AblyMacDemo/Pods/Ably/Source/Private/ARTOSReachability.m:96
#8  0x0000000100532844 in _dispatch_call_block_and_release ()
#9  0x0000000100533826 in _dispatch_client_callout ()
#10 0x000000010053add7 in _dispatch_lane_serial_drain ()
#11 0x000000010053bb90 in _dispatch_lane_invoke ()
#12 0x0000000100548fe0 in _dispatch_workloop_worker_thread ()
#13 0x00000001005c1361 in _pthread_wqthread ()
#14 0x00000001005c049b in start_wqthread ()