[turbofan] Fixed segmentation fault while printing TurboFan node

Graph trimmer can set Input as null and subsequent
printing call could cause segmentation fault.

R=bmeurer@chromium.org, jarin@chromium.org

Review-Url: https://codereview.chromium.org/2134443002
Cr-Commit-Position: refs/heads/master@{#37658}

AUTHORS

@@ -52,6 +52,7 @@ Chris Nardi <hichris123@gmail.com>
 Christopher A. Taylor <chris@gameclosure.com>
 Daniel Andersson <kodandersson@gmail.com>
 Daniel James <dnljms@gmail.com>
+Deon Dior <diaoyuanjie@gmail.com>
 Douglas Crosher <dtc-v8@scieneer.com>
 Dusan Milosavljevic <dusan.m.milosavljevic@gmail.com>
 Erich Ocean <erich.ocean@me.com>

src/compiler/node.cc

@@ -369,7 +369,11 @@ std::ostream& operator<<(std::ostream& os, const Node& n) {
     os << "(";
     for (int i = 0; i < n.InputCount(); ++i) {
       if (i != 0) os << ", ";
-      os << n.InputAt(i)->id();
+      if (n.InputAt(i)) {
+        os << n.InputAt(i)->id();
+      } else {
+        os << "null";
+      }
     }
     os << ")";
   }