Initialize unsigned char variables

[radarhere]

https://github.com/python-pillow/Pillow/actions/runs/3752682144/jobs/6375115871#step:7:141

src/libImaging/Quant.c: In function ‘ImagingQuantize’:
src/libImaging/Quant.c:1720:29: warning: ‘b’ may be used uninitialized in this function [-Wmaybe-uninitialized]
1720 | unsigned char r, g, b;
| ^
src/libImaging/Quant.c:1720:26: warning: ‘g’ may be used uninitialized in this function [-Wmaybe-uninitialized]
1720 | unsigned char r, g, b;
| ^
src/libImaging/Quant.c:1720:23: warning: ‘r’ may be used uninitialized in this function [-Wmaybe-uninitialized]
1720 | unsigned char r, g, b;
| ^

[radarhere]

For the record - they were always initialized before use, just not in a way that the compiler could detect.