title: App crashes when access the length of [NSData dataWithContentsOfURL:url]
body: <p>Here is the problem:</p>
  
  <p>The app crashes at the 2nd line, and I got EXC_BAD_ACCESS exception.</p>
  
  <pre><code>NSURL *url = [NSURL URLWithString:@"..."];
  
  NSError *error = nil;
  NSData *data = [NSData dataWithContentsOfURL:url 
            options:NSDataReadingMapped 
              error:&amp;error];
  NSLog(@"Error: %@", error);
  NSLog(@"%@", [data length]);
  </code></pre>
  
  <p>I got:</p>
  
  <pre><code>Error: (null)
  Program received signal:  “EXC_BAD_ACCESS”.
  </code></pre>
  
  <p>Any idea?</p>