There is a small memory leak when cloning strings in ImageMagick 6.3.5-9.  The
patch is already in ImageMagick 6.3.6-0 scheduled for release on September 29. 
To patch ImageMagick 6.3.5-9, remove

   *destination=(char *) NULL; 

in CloneString() of magick/string.c.  The results should look like this:

  if (source == (const char *) NULL)
    {
      if (*destination != (char *) NULL)
        *destination=DestroyString(*destination);
      return(*destination);
    }
  if (*destination == (char *) NULL)
    {
      *destination=AcquireString(source);
      return(*destination);
    }
  length=strlen(source);
  if (~length < MaxTextExtent)
    ThrowFatalException(ResourceLimitFatalError,"UnableToAcquireString");
  *destination=(char *) ResizeQuantumMemory(*destination,length+MaxTextExtent,
    sizeof(*destination));