ENH: additional protection against zero-sized graph offset lists

- for metis-like graphs there is no guarantee that a zero-sized graph
  has an offsets list with size 1 or size 0, so always use

     numCells = max(0, xadj.size()-1)

  this was already done in most places, but missed in the
  decomposeGeneral method

STYLE: use sumOp<label>() instead of plusOp<label>()

applications/utilities/preProcessing/setExprFields/setExprFields.C

@@ -208,8 +208,8 @@ bool setField
 
     doCorrectBoundaryConditions(ctrl.correctBCs, output);
 
-    const label numTotal = returnReduce(output.size(), plusOp<label>());
-    reduce(numValuesChanged, plusOp<label>());
+    const label numTotal = returnReduce(output.size(), sumOp<label>());
+    reduce(numValuesChanged, sumOp<label>());
 
     if (numValuesChanged == numTotal)
     {