message: Prevent segfault in UpdateNode
  If src_context is null because the src is null, the error message will cause a segfault trying to access src->name(). Also, src_context was used before it was checked to be null.
diff: @@ -783,13 +783,13 @@ class SymbolicShapeRefiner {
  int src_output = fanin.port_id;
  const NodeDef* src = fanin.node;
  NodeContext* src_ctx = GetNodeContext(src);
  - InferenceContext* src_ic = src_ctx->inference_context.get();
  if (src_ctx == nullptr) {
  return errors::FailedPrecondition(
  - "Input ", dst_input, " ('", src->name(), "') for '", node->name(),
  + "Input ", dst_input, " for '", node->name(),
  "' was not previously added to SymbolicShapeRefiner.");
  }
  + InferenceContext* src_ic = src_ctx->inference_context.get();
  if (src_output >= src_ic->num_outputs()) {
  return errors::OutOfRange("src_output = ", src_output,
  ", but num_outputs is only ",
  