compute: fix crashes
compute: fix crashes

```
/Users/dcrossland/fonts/ofl/corben/Corben-Bold.ttf
AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvXxYyZz
Traceback (most recent call last):
  File
"src/github.com/googlefonts/fontbakery/tools/compute_font_weight_and_wid
th.py", line 254, in <module>
    main()
  File
"src/github.com/googlefonts/fontbakery/tools/compute_font_weight_and_wid
th.py", line 124, in main
    properties.append(get_width(fontfile))
  File
"src/github.com/googlefonts/fontbakery/tools/compute_font_weight_and_wid
th.py", line 203, in get_width
    print font.getsize(TEXT)
  File "/usr/local/lib/python2.7/site-packages/PIL/ImageFont.py", line
141, in getsize
    size, offset = self.font.getsize(text)
IOError: invalid reference
```

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