message: Fix possible segfault with ShapeHandle.
diff: @@ -306,7 +306,7 @@ class InferenceContext {
  // idx can be negative for an offset from end of dimensions.
  // idx must be in the range [-1 * s.rank, s.rank).
  DimensionHandle Dim(ShapeHandle s, int64 idx) {
  - if (s->rank_ == kUnknownRank) {
  + if (!s.Handle() || s->rank_ == kUnknownRank) {
  return UnknownDim();
  }
  return DimKnownRank(s, idx);
  