My SVG avatar appears to be causing a 500

```
$ curl -I "https://chat.indieweb.org/img.php?url=https%3A%2F%2Fiambismark.net%2Favatar.svg&sig=a5a2e6906c3379549bf588b963d8365b8f4088ec24084d0bb34cb8b338bb5042"
HTTP/2 500
server: nginx/1.14.0
date: Thu, 26 Jul 2018 01:37:03 GMT
content-type: text/html; charset=UTF-8
x-powered-by: PHP/5.6.36-1+ubuntu16.04.1+deb.sury.org+1
```