Fix warning from getimagesize called on empty files (bug #848595)

If there are image artefacts with zero size files, getimagesize
outputs unfriendly "Read error!" warnings into the logs.

Change-Id: I9cd8fcf990709da3b34b5530235a065253ce46df
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>

htdocs/lib/file.php

@@ -519,7 +519,7 @@ function get_dataroot_image_path($path, $id, $size=null) {
 
         // Image is not available in this size. If there is a base image for
         // it, we can make one however.
-        if (is_readable($originalimage)) {
+        if (is_readable($originalimage) && filesize($originalimage)) {
 
             $imageinfo = getimagesize($originalimage);
             $originalmimetype = $imageinfo['mime'];