Error when including image with bogus resolution

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Hi all,

(Context: https://tex.stackexchange.com/q/734919)

If an image has a way too small resolution, pdfTeX includes it
incorrectly and issues a warning:

    pdfTeX warning: pdftex: arithmetic: number too big

Instead of giving an unclear warning and failing to properly include the
image, it would be better to treat this like a too large resolution and
to issue a warning and reset the resolution.

Test file:

    %% Generate with ImageMagick:
    %%     for n in $(seq 1 6); do
    %%         convert -units PixelsPerInch -size 1000x1000 xc:#ff0000 -den=
sity $n test-$n.jpg
    %%     done
    \pdfximage{test-1.jpg}\pdfrefximage\pdflastximage\eject
    \pdfximage{test-2.jpg}\pdfrefximage\pdflastximage\eject
    \pdfximage{test-3.jpg}\pdfrefximage\pdflastximage\eject
    \pdfximage{test-4.jpg}\pdfrefximage\pdflastximage\eject
    \pdfximage{test-5.jpg}\pdfrefximage\pdflastximage\eject
    \pdfximage{test-6.jpg}\pdfrefximage\pdflastximage\bye

Patch attached. ("resolution_ratio" is a terrible name; hopefully you
can think of something better)

Thanks,
-- Max

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