Mark the `SDAsyncBlockOperation` safe using the lock and correct override method

SDWebImage/Private/SDAsyncBlockOperation.m

@@ -35,7 +35,9 @@
 }
 
 - (void)start {
+    @synchronized (self) {
         if (self.isCancelled) {
+            self.finished = YES;
             return;
         }
         
@@ -46,24 +48,42 @@
         if (self.executionBlock) {
             self.executionBlock(self);
         } else {
-        [self complete];
+            self.executing = NO;
+            self.finished = YES;
+        }
     }
 }
 
 - (void)cancel {
+    @synchronized (self) {
         [super cancel];
         if (self.isExecuting) {
-        [self complete];
+            self.executing = NO;
+            self.finished = YES;
+        }
     }
 }
 
+ 
 - (void)complete {
-    [self willChangeValueForKey:@"isExecuting"];
-    [self willChangeValueForKey:@"isFinished"];
+    @synchronized (self) {
+        if (self.isExecuting) {
             self.executing = NO;
             self.finished = YES;
-    [self didChangeValueForKey:@"isExecuting"];
+        }
+    }
+ }
+
+- (void)setFinished:(BOOL)finished {
+    [self willChangeValueForKey:@"isFinished"];
+    _finished = finished;
     [self didChangeValueForKey:@"isFinished"];
 }
 
+- (void)setExecuting:(BOOL)executing {
+    [self willChangeValueForKey:@"isExecuting"];
+    _executing = executing;
+    [self didChangeValueForKey:@"isExecuting"];
+}
+
 @end