Found a typo and an error in return value of graphviz_filter_file_download

First I'd like to thank you for providing this awesome module.

I found a typo in function graphviz_filter_file_download(): there a function named file_crieate_path() is called, but he function name should be file_create_path()

There is also an error in the return value of the function graphviz_filter_file_download(). If '/graphviz' is not a part of the filepath the function returns -1. But this denies the access to this file. I am using private download method in my Drupal site and so after I installed graphviz filter I do not see images on image nodes anymore, because when the all file_download hooks get invoked, graphviz_filter_file_download() denies access to the images returning -1.

As you can see in the api definition of hook_file_download (of Drupal 6, in the description of Drupal 5 api this information is not so clear), the return value should be NULL if the file is not controlled by the current module.

I append a short patch which fixes the typo and the error with the return value. Please apply the patch from the site's module directory (e.g. /var/www/drupal/sites/all/modules/)