MDL-50690 themes: Theme cache SVG images are not created.

theme/image.php

@@ -167,6 +167,10 @@ if (!$usesvg) {
             }
             send_uncached_image($imagefile);
             exit;
+        } else {
+            /* We cannot serve the SVG file this time and there is no alternative, so prevent future
+               requests from failing to find the image when they can support SVG. */
+            image_not_found();
         }
     }
 }