Fix decoding interlaced images with height or width <= 2

[fhanau]

Fixes decoding of interlaced images with height or width <= 2, which previously triggered an assertion as the iterator did not detect which passes are present in an image properly.

[fhanau]

Test file created using convert -size 2x2 canvas:white -interlace PNG issue-175.png:

https://user-images.githubusercontent.com/12156995/59851836-7ecf3700-9365-11e9-893f-92f838ec830c.png

[shssoichiro]

Thanks! Would you mind adding a regression test using that sample image? You can see how the other regression tests are set up under tests/regression.rs.

[fhanau]

Done!

[shssoichiro]

Thanks!

[shssoichiro]

Looks like rustup is broken at the moment, so I'm going to merge this despite the red build.