e.g. cairo_pixels = g_malloc (height * cairo_stride); ... guchar *q = cairo_pixels; ... MULT(q[0], p[2], p[3], t1); => crash
e.g.
MULT(q[ 0], p[2], p[3], t1);
cairo_pixels = g_malloc (height * cairo_stride);
...
guchar *q = cairo_pixels;
...
=> crash