Helps #6639

When Pillow reads a Gimp palette file into palette bytes, the palette bytes are not populated by the data from the start. Rather, the loop index continues increasing when reaching header fields or a comment, making the final positions of the entries somewhat arbitrary. So this only increases the loop index when an entry is found.

This PR also pulls in the idea from #5552 that palettes can be less than 256 entries - once the end of the Gimp palette file is reached, let's stop populating the palette bytes.

This is similar to #6640, but that PR seeks read additional bytes from the file, limited by new class variables. I'm reluctant to go there, so this PR helps by at least solving the underlying logic problem.