Fix comparison warnings

[radarhere]

https://github.com/python-pillow/Pillow/actions/runs/23992626025/job/69982526309#step:5:39215

  src/libImaging/FliDecode.c: In function 'ImagingFliDecode':
  src/libImaging/FliDecode.c:52:29: warning: comparison of integer expressions of different signedness: 'Py_ssize_t' {aka 'long int'} and 'long unsigned int' [-Wsign-compare]
     52 |     if (bytes + (bytes % 2) < framesize) {
        |                             ^
  src/libImaging/FliDecode.c:262:36: warning: comparison of integer expressions of different signedness: 'long unsigned int' and 'Py_ssize_t' {aka 'long int'} [-Wsign-compare]
    262 |         if (advance < 0 || advance > bytes) {
        |                                    ^

We know that bytes is positive, thanks to

Pillow/src/libImaging/FliDecode.c

Lines 41 to 43 in f5ab7bb

	if (bytes < 4) {
	return 0;
	}

and

Pillow/src/libImaging/FliDecode.c

Lines 75 to 78 in f5ab7bb

	if (bytes < 10) {
	state->errcode = IMAGING_CODEC_OVERRUN;
	return -1;
	}