r600g/sb: Skip empty ALU clause while scheduling

Fixes assert triggered by
ext_transform_feedback-intervening-read output use_gs
piglit test.

Signed-off-by: Glenn Kennard <glenn.kennard@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/gallium/drivers/r600/sb/sb_sched.cpp

@@ -825,6 +825,9 @@ void post_scheduler::init_regmap() {
 
 void post_scheduler::process_alu(container_node *c) {
 
+	if (c->empty())
+		return;
+
 	ucm.clear();
 	alu.reset();