title: release CFString
body: <p>I use this code to get the last name of ABPerson </p>
  
  <pre><code>CFStringRef lastNameRef = ABRecordCopyValue((ABRecordRef)personRecordRef, kABPersonLastNameProperty);
  NSString *friendLastName = (NSString*)lastNameRef;
  CFRelease(lastNameRef);
  </code></pre>
  
  <p>it work fine when the value of last name is not equal to NULL 
  but when the this value is NULL the application crash at the third line because I try to relese NULL </p>
  
  <p>the question is witch is the best way to releasing the CFString in this case without causing the crash of the application  </p>