Attempt to fix #1883 by using a weakSelf reference to remove from callbackBlocks…

… (on the barrierQueue)

@@ -223,8 +223,9 @@ typedef NSMutableDictionary<NSString *, id> SDCallbacksDictionary;
 }
 
 - (void)reset {
+    __weak typeof(self) weakSelf = self;
     dispatch_barrier_async(self.barrierQueue, ^{
-        [self.callbackBlocks removeAllObjects];
+        [weakSelf.callbackBlocks removeAllObjects];
     });
     self.dataTask = nil;
     self.imageData = nil;