Fix the imagecropauto() threshold color error message - #25385

imagecropauto() in threshold mode reports "must be greater than or equal to 0" for a non-negative colour that is not an index of the palette. Report the palette bound separately.

$p = imagecreate(6, 6);
imagecolorallocate($p, 255, 255, 255);
imagecolorallocate($p, 10, 10, 10);
imagecropauto($p, IMG_CROP_THRESHOLD, 0.5, imagecolorstotal($p));

Before: ValueError: imagecropauto(): Argument #4 ($color) must be greater than or equal to 0 when using the threshold mode

After: ValueError: imagecropauto(): Argument #4 ($color) must be a valid color index of the image when using the threshold mode