[turbofan] Fix segfault when using --trace-turbo.

src/compiler/graph-visualizer.cc

Index: src/compiler/graph-visualizer.cc
diff --git a/src/compiler/graph-visualizer.cc b/src/compiler/graph-visualizer.cc
index 132442d6add88069a8bb9b146c0c6bd7083a2a0a..74b1eaef10a4c9db28ca49e0254ebbfc3240605c 100644
--- a/src/compiler/graph-visualizer.cc
+++ b/src/compiler/graph-visualizer.cc
@@ -711,6 +711,7 @@ void GraphC1Visualizer::PrintLiveRanges(const char* phase,
 
 void GraphC1Visualizer::PrintLiveRangeChain(TopLevelLiveRange* range,
                                             const char* type) {
+  if (range == nullptr || range->IsEmpty()) return;
   int vreg = range->vreg();
   for (LiveRange* child = range; child != nullptr; child = child->next()) {
     PrintLiveRange(child, type, vreg);