Fixing overflow issue that appeared with GCC compilers.

Thanks to Michael Stephens for the report and fix.

Change-Id: Ibe68819a81b011d3e3b0d8fefc2bc122834e8602

IO/Geometry/vtkMultiBlockPLOT3DReaderInternals.cxx

@@ -375,7 +375,7 @@ long vtkMultiBlockPLOT3DReaderInternals::CalculateFileSizeForBlock(int precision
 {
   long size = 0;
   // x, y, (z)
-  int npts = 1;
+  long npts = 1;
   for (int i=0; i<ndims; i++)
     {
     npts *= gridDims[i];