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

BUG=

Review URL: https://codereview.chromium.org/1314703005

Cr-Commit-Position: refs/heads/master@{#30584}

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);