Cast int before potentially exceeding INT_MAX

[radarhere]

Resolves #8401

tablen is an integer

Pillow/src/libImaging/Sgi.h

Line 22 in b557876

int tablen;

and before we set it to im->bands * im->ysize, we check that the value will be less than INT_MAX

Pillow/src/libImaging/SgiRleDecode.c

Lines 170 to 172 in b557876

	if (im->xsize > INT_MAX / im->bands || im->ysize > INT_MAX / im->bands) {
	state->errcode = IMAGING_CODEC_MEMORY;
	return -1;

but we then temporarily multiply it by 8, which might exceed INT_MAX.

Pillow/src/libImaging/SgiRleDecode.c

Lines 181 to 186 in b557876

	c->tablen = im->bands * im->ysize;
	/* below, we populate the starttab and lentab into the bufsize,
	each with 4 bytes per element of tablen
	Check here before we allocate any memory
	*/
	if (c->bufsize < 8 * c->tablen) {

So this PR casts it to int64_t before applying the multiplication.