pattern: Fix _gradient_is_opaque() for zero stops

_gradient_is_opaque() previously returned TRUE for gradient with
no stops, triggering a false optimization in _cairo_gstate_mask().

Fixes test/gradient-zero-stops-mask

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

src/cairo-pattern.c

@@ -1795,6 +1795,9 @@ _gradient_is_opaque (const cairo_gradient_pattern_t *gradient)
 {
     unsigned int i;
 
+    if (gradient->n_stops == 0)
+	return FALSE;
+
     for (i = 0; i < gradient->n_stops; i++)
 	if (! CAIRO_COLOR_IS_OPAQUE (&gradient->stops[i].color))
 	    return FALSE;