BUG: vtk::internalWriter::writeProcIDs could block in parallel

- could be triggered if running in parallel, but requesting procIDs
  to be written with a non-parallel version of the writer.

src/conversion/vtk/output/foamVtkInternalWriter.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2017-2018 OpenCFD Ltd.
+    \\  /    A nd           | Copyright (C) 2017-2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -665,9 +665,9 @@ void Foam::vtk::internalWriter::writeCellIDs()
 
 bool Foam::vtk::internalWriter::writeProcIDs()
 {
-    if (!Pstream::parRun())
+    if (!parallel_)
     {
-        // Skip serial output (meaningless)
+        // Disabled in serial output (meaningless)
         return false;
     }