Merge pull request AFNetworking#1545 from pavolas/cancel_bug

Fix a non-deterministic failure in [AFURLConnectionOperation cancel]

AFNetworking/AFURLConnectionOperation.m

@@ -439,10 +439,6 @@ - (void)operationDidStart {
     dispatch_async(dispatch_get_main_queue(), ^{
         [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingOperationDidStartNotification object:self];
     });
-
-    if ([self isCancelled]) {
-        [self finish];
-    }
 }
 
 - (void)finish {
@@ -474,9 +470,15 @@ - (void)cancelConnection {
     }
     NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorCancelled userInfo:userInfo];
 
-    if (![self isFinished] && self.connection) {
-        [self.connection cancel];
-        [self performSelector:@selector(connection:didFailWithError:) withObject:self.connection withObject:error];
+    if (![self isFinished]) {
+        if (self.connection) {
+            [self.connection cancel];
+            [self performSelector:@selector(connection:didFailWithError:) withObject:self.connection withObject:error];
+        } else {
+            // If 'cancel' is called immediately after 'start', there is a race condition where self.connection has not yet been instantiated. We still want the callback to go through and the error to be set.
+            self.error = error;
+            [self finish];
+        }
     }
 }