meet crash

[lazyjean]

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Issue Info

Info	Value
Platform Name	iOS
Platform Version	10.0.2
SDWebImage Version	4.1.0
Integration Method	cocoapods
Xcode Version	Xcode8
Repro rate	only once
Repro with our demo prj	NO

Issue Description and Steps

Crashed: NSOperationQueue 0x1702225a0 :: NSOperation 0x170e44950 (QOS: DEFAULT)
0 libobjc.A.dylib 0x18c3db324 cache_getImp + 4
1 libobjc.A.dylib 0x18c3d0c80 lookUpImpOrForward + 284
2 libobjc.A.dylib 0x18c3db258 _objc_msgSend_uncached + 56
3 SDWebImage 0x1008a2a10 -[SDWebImageDownloaderOperation URLSession:dataTask:didReceiveData:] (SDWebImageDownloaderOperation.m:301)
4 SDWebImage 0x1008a0868 -[SDWebImageDownloader URLSession:dataTask:didReceiveData:] (SDWebImageDownloader.m:304)
5 CFNetwork 0x18e141634 __67-[NSURLSession delegate_dataTask:didReceiveData:completionHandler:]_block_invoke.242 + 36
6 Foundation 0x18e4517e4 NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK + 16
7 Foundation 0x18e396358 -[NSBlockOperation main] + 96
8 Foundation 0x18e386954 -[__NSOperationInternal _start:] + 620
9 Foundation 0x18e453b90 __NSOQSchedule_f + 228
10 libdispatch.dylib 0x18c8191c0 _dispatch_client_callout + 16
11 libdispatch.dylib 0x18c827444 _dispatch_queue_serial_drain + 928
12 libdispatch.dylib 0x18c81c9a8 _dispatch_queue_invoke + 652
13 libdispatch.dylib 0x18c82938c _dispatch_root_queue_drain + 572
14 libdispatch.dylib 0x18c8290ec _dispatch_worker_thread3 + 124
15 libsystem_pthread.dylib 0x18ca212c8 _pthread_wqthread + 1288
16 libsystem_pthread.dylib 0x18ca20db4 start_wqthread + 4

[dreampiggy]

Duplicated issue related to imageData multithreads crash : #1998 PR: #2011