Bug 1016482. Fix broken createPattern test. r=jgraham

Per spec, createPattern does not throw an InvalidStateError if the image's dimensions have been determined. The "broken" image this test uses has a valid headed and dimensions, but invalid image data so it does not cause InvalidStateError to be thrown, per spec; instead it causes null to be returned.

Differential Revision: https://phabricator.services.mozilla.com/D63030