message: Don't allocate the SparseMatrix if there is an extension.
diff: @@ -233,7 +233,7 @@ void ParBilinearForm::AssembleSharedFaces(int skip_zeros)
  void ParBilinearForm::Assemble(int skip_zeros)
  {
  - if (mat == NULL && fbfi.Size() > 0)
  + if (!ext && mat == NULL && fbfi.Size() > 0)
  {
  pfes->ExchangeFaceNbrData();
  pAllocMat();
  