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

src/compiler/graph-visualizer.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/graph-visualizer.h"
 
 #include <sstream>
 #include <string>
 
 #include "src/code-stubs.h"
@@ skipping 693 matching lines @@
   }
 
   for (auto range : data->live_ranges()) {
     PrintLiveRangeChain(range, "object");
   }
 }
 
 
 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);
   }
 }
 
 
 void GraphC1Visualizer::PrintLiveRange(LiveRange* range, const char* type,
                                        int vreg) {
   if (range != NULL && !range->IsEmpty()) {
@@ skipping 102 matching lines @@
         os << "#" << SafeId(i) << ":" << SafeMnemonic(i);
       }
       os << ")" << std::endl;
     }
   }
   return os;
 }
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8