Bug 737909 - Address Joe's review comment; r=joe

gfx/cairo/cairo/src/cairo-quartz-surface.c

@@ -441,7 +441,7 @@ _cairo_quartz_cairo_operator_to_quartz_composite (cairo_operator_t op)
 	case CAIRO_OPERATOR_HSL_LUMINOSITY:
         default:
 	    assert (0);
-	    return 0;
+	    return kPrivateCGCompositeClear;
     }
 }