r600g/sb: Move variable dereference after null check.

Fixes "Deference before null check" defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Vadim Girlin <vadimgirlin@gmail.com>

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

@@ -395,11 +395,12 @@ void ra_init::color_bs_constraint(ra_constraint* c) {
 
 	for (vvec::iterator I = vv.begin(), E = vv.end(); I != E; ++I) {
 		value *v = *I;
-		sel_chan gpr = v->get_final_gpr();
 
 		if (!v || v->is_dead())
 			continue;
 
+		sel_chan gpr = v->get_final_gpr();
+
 		val_set interf;
 
 		if (v->chunk)