API: Fix imageinfo iiurlheight on audio files

API: Fix imageinfo iiurlheight on audio files

When iiurlheight is given without iiurlwidth, we want to limit the
thumbnail by just the height. MediaWiki's file classes don't really
support this, though, so we pass the image's full width as the limiting
width to achieve the same effect.

However, this fails for audio files where $file->getWidth() returns 0,
but we can still get a thumbnail of a placeholder image. In that
situation, we should just choose an arbitrary non-zero limiting width.

Change-Id: I4318a06a96265d39e39e90cc706d49a1c3b6e8e3