Wrong Content-Length in 'readfile_accel' if file does not exist.

Non-Files API

1. First test non file api files, make a test script:

   <?php

   require('config.php');

   require_once($CFG->libdir . '/filelib.php');

   readfile_accel($CFG->libdir.'/thirdpartylibs.xml', 'text/xml', 1);

2. Call this using curl:

   curl -sv http://moodle.local/test.php > /dev/null

3. Note the content length (e.g. 8918)
4. Test is also works fine with a range request

   curl -sv -H "Range: bytes=0-33" http://moodle.local/test.php

   Content-Length: 34

5. Now change the test to a file path that doesn't exist

   <?php

   require('config.php');

   require_once($CFG->libdir . '/filelib.php');

   readfile_accel($CFG->libdir.'/nope.xml', 'text/xml', 1);

6. Fetch the file again:

   curl -sv [http://moodle.local/test.php]

   1. Confirm that there was no Content-Length specified
7. Fetch the same page in your browser
   1. Confirm that the Moodle exception was shown

Files API

1. Log in to Moodle as admin
2. View the Site home
3. Turn editing on
4. Create a new label and upload a PNG image to it
5. Save changes
6. Run the following DB query in your preferred DB client:

   select * from mdl_files where filearea = 'mod_label' order by id desc limit 2;

7. Note the contenthash, and the filesize for later, e.g.

   4e1a025e5a09cba870cfa394faa8270524170f1e

8. Right click on the image, and copy the image address
9. Fetch it using curl:

   curl -sv [url] > /dev/null

   1. Confirm that the Content-Length matches the value you noted earlier
10. Now fetch just the first 32 bytes:

    curl -sv -H "Range: bytes=0-31" [url] --output -

    ## Confirm that the new Content-Length is 32

    1. Confirm that the output started with the following

       �PNG

11. Find the file in your moodledata directory using the contenthash from earlier
    For example, if the hash was 4e1a025e5a09cba870cfa394faa8270524170f1e you'll find it in

    /[datadir]/4e/1a/4e1a025e5a09cba870cfa394faa8270524170f1e

12. Rename the file by adding an extension to it:

    /[datadir]/4e/1a/4e1a025e5a09cba870cfa394faa8270524170f1e.bak

13. Open the URL in your browser again and force refresh of the page
    1. Confirm that you were shown an error that that the file could not be found and
    2. Confirm that the error message included the Error code: storedfilecannotreadfile
14. Perform a range request again (same as before)

    curl -sv -H "Range: bytes=0-31" [url] --output -

    1. Confirm that there was no Content-Length
    2. Confirm that a load of HTML was output and not just the first 32 bytes