Ensure palette string matches RGB mode

[radarhere]

Resolves #4830

The image from that issue has a BGR;15 palette. So when PngImagePlugin tries to calculate the number of colors to use when saving for show(),

Pillow/src/PIL/PngImagePlugin.py

Line 1195 in 384a4bf

colors = max(min(len(im.palette.getdata()[1]) // 3, 256), 1)

the assumption, that it can divide by 3 because this is RGB, is false.

This PR adds a change to Image, so that when it sets the palette mode to RGB, there is actually RGB data in the palette string.

[radarhere]

The first commit is failing, as another test complains that the palette is now longer than it should be.

This is because I'm calling getpalette(), which has a fixed palette length.

The C layer ImagingPaletteInstance doesn't actually have a palette length, so instead, I've returned the length from putpalette and used that in Python to trim the output.

[radarhere]

The CI Fuzz failure is related. See https://github.com/radarhere/Pillow/actions/runs/951376400 from just a comment change in C.

[hugovk]

The CI Fuzz failure is related. See radarhere/Pillow/actions/runs/951376400 from just a comment change in C.

You mean unrelated?

[radarhere]

Yes, unrelated :p

[radarhere]

I've created google/oss-fuzz#5950 to fix the CI Fuzz failure.

[radarhere]

PR has been merged, and CI Fuzz is working again.