Segfault for background image with unknown extension

[bram]

Hi! More segfaults from me.

Following code segfaults on svn trunk:

Shoes.app do
  background "foo.bar"
end

Or any other background filename with an extension that is not a gif, jpeg or png (regardless of whether the file "foo.bar" exists or not). I know that this code does not make sense, but at least it shouldn't segfault, right?

[bram]

Oh, forgot to mention, this is on Linux -- I don't know about other platforms.

[why]

Many revisions ago -- in [414] -- an unsupported image error was added to address this. Thanks!