crash in _fill_xrgb32_lerp_opaque_spans

Hello,

First of all, million thanks for this beautiful library!
I have encountered a crash in _fill_xrgb32_lerp_opaque_spans (cairo 1.14.0,
pixman 0.32.6):

I had to replace if (len--) to if (len-- > 0) to let our program run.

static cairo_status_t
_fill_xrgb32_lerp_opaque_spans (void *abstract_renderer, int y, int h,
                const cairo_half_open_span_t *spans, unsigned num_spans)
{
    cairo_image_span_renderer_t *r = abstract_renderer;

    if (num_spans == 0)
    return CAIRO_STATUS_SUCCESS;

    if (likely(h == 1)) {
    do {
        uint8_t a = spans[0].coverage;
        if (a) {
        int len = spans[1].x - spans[0].x; // = -1 causing a crash
        uint32_t *d = (uint32_t*)(r->u.fill.data + r->u.fill.stride*y +
spans[0].x*4);
        if (a == 0xff) {
            if (len > 31) {
            pixman_fill ((uint32_t *)r->u.fill.data, r->u.fill.stride /
sizeof(uint32_t), 32,
                     spans[0].x, y, len, 1, r->u.fill.pixel);
            } else {
            uint32_t *d = (uint32_t*)(r->u.fill.data + r->u.fill.stride*y +
spans[0].x*4);
            while (len-- > 0)
                *d++ = r->u.fill.pixel;
            }
        } else while (len-- > 0) { // crash len being negative (was -1 at
the line 2238: int len = spans[1].x - spans[0].x)
            *d = lerp8x4 (r->u.fill.pixel, a, *d);
            d++;
        }
        }
        spans++;
....

With best regards,
Ilya

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