silence warnings about cases, that cannot happen

(cherry picked from commit 60bf26bad9)

src/fix_controller.cpp

@@ -197,7 +197,7 @@ void FixController::end_of_step()
 
   // invoke compute if not previously invoked
 
-  double current;
+  double current = 0.0;
 
   if (pvwhich == COMPUTE) {
     if (pvindex == 0) {

src/imbalance_var.cpp

@@ -59,8 +59,9 @@ void ImbalanceVar::compute(double *weight)
 {
   if (_id >= 0) {
     const int all = _lmp->group->find("all");
-    const int nlocal = _lmp->atom->nlocal;
+    if (all < 0) return;
 
+    const int nlocal = _lmp->atom->nlocal;
     double *val = new double[nlocal];
     _lmp->input->variable->compute_atom(_id,all,val,1,0);
     for (int i = 0; i < nlocal; ++i) weight[i] *= val[i];

src/variable.cpp

@@ -898,7 +898,7 @@ double Variable::compute_equal(int ivar)
     error->all(FLERR,"Variable has circular dependency");
   eval_in_progress[ivar] = 1;
 
-  double value;
+  double value = 0.0;
   if (style[ivar] == EQUAL) value = evaluate(data[ivar][0],NULL);
   else if (style[ivar] == INTERNAL) value = dvalue[ivar];
   else if (style[ivar] == PYTHON) {