Allow zero DPI and only throw at EOF when not enough data

[JimBobSquarePants]

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Description

Fixes #2315

The image has two issues.

1. Zero density unit values in the JFIF header.
2. Erroneous marker data at the end of the stream following the image data.

I've relaxed the decoder to cater for these cases.

cc\ @br3aker

[br3aker]

As we want to know if image was decoded at this point - we could use Converted of SpectralConverter as it's called right after baseline image is decoded.

Two problems:

1. It's protected, not really a problem creating a public getter though
2. It's not set for progressive images as those are lazy-converted during pixel buffer -> Image<T> step which won't be called until parsing loop is finished, not the wisest solution from me

I'd probably refactor spectral converter so it could better describe its state.

[JimBobSquarePants]

Yeah. This felt a little hacky. State matching would be better.

[br3aker]

Looks good!