Bug 737909 - Fix opt -Werror build with clang; r=joe

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

@@ -441,6 +441,7 @@ _cairo_quartz_cairo_operator_to_quartz_composite (cairo_operator_t op)
 	case CAIRO_OPERATOR_HSL_LUMINOSITY:
         default:
 	    assert (0);
+	    return 0;
     }
 }