Propagate bug fixes to other variant of Averaging::addSampleInternal

Bugs in the variance handling (calculation indexing and protecting
when it is null) were discovered and fixed during the extension of the
Averaging class into the LoMachSolver.  But, these were present in the
variant used on the compressible path also, so we fix them too.

src/averaging.cpp

@@ -347,7 +347,10 @@ void Averaging::addSampleInternal(GasMixture *mixture) {
 
     const double *d_inst = inst->Read();
     double *d_mean = mean->ReadWrite();
-    double *d_vari = vari->ReadWrite();
+    double *d_vari = nullptr;
+    if (vari != nullptr) {
+      d_vari = vari->ReadWrite();
+    }
 
     // Get mixture class pointer for use on device
     //
@@ -418,7 +421,7 @@ void Averaging::addSampleInternal(GasMixture *mixture) {
 
         // Covariances second (i.e., off-diagonal components, if any)
         for (int i = d_vari_start; i < d_vari_start + d_vari_components - 1; i++) {
-          for (int j = d_vari_start + 1; j < d_vari_start + d_vari_components; j++) {
+          for (int j = i + 1; j < d_vari_start + d_vari_components; j++) {
             val = d_vari[n + vari_index * dof];
             delta_i = (nUp[i] - mean_state[i]);
             delta_j = (nUp[j] - mean_state[j]);