ENH: refine resizing of zero-sized DynamicList/DynamicField

- since List is being used to manage the storage content for
  DynamicList, it needs to free old memory for zero-sized lists first.

  Consider this case (slightly exaggerated):

      line 0:  DynamicList<label> list;
      line 1:  list.reserve(100000);
      line 2:  list.reserve(200000);

  After line 0:
     - list has size=0, capacity=0 and data=nullptr

  After line 1:
     - list has size=0, capacity=1e+5 and data != nullptr

  After line 2:
     - list has size=0, capacity=2e+5 and data != nullptr

  ---

  The internal resizing associated with line 1 corresponds to what the
  List resize would naturally do. Namely allocate new storage, copy/move
  any overlapping elements (in this case none) before freeing the old
  storage and replacing with new storage.

  Applying the same resizing logic for line 2 means, however, that the
  old memory (1e5) and new memory (2e5) are temporarily both
  accessible - leading to an unnecessary memory peak.

  Now: if there is no overlap, just remove old memory first.

src/OpenFOAM/containers/Lists/List/List.C

@@ -46,12 +46,25 @@ void Foam::List<T>::doResize(const label len)
     if (len > 0)
     {
         // With sign-check to avoid spurious -Walloc-size-larger-than
-        T* nv = new T[len];
-
         const label overlap = min(this->size_, len);
 
-        if (overlap)
+        if (!overlap)
         {
+            // Can discard old content before allocating new storage.
+            // - when used as storage for DynamicList, it is possible to have
+            //   a zero-sized List with a non-null data pointer.
+
+            if (this->v_) delete[] this->v_;
+            this->size_ = len;
+            this->v_ = new T[len];
+        }
+        else
+        {
+            // Recover old (overlapping) content when resizing
+
+            T* nv = new T[len];
+
+            // ie, std::copy(this->v_, this->v_ + overlap, nv);
             #ifdef USEMEMCPY
             if (is_contiguous<T>::value)
             {
@@ -69,12 +82,12 @@ void Foam::List<T>::doResize(const label len)
                     nv[i] = std::move(vp[i]);
                 }
             }
-        }
 
-        clear();
+            delete[] this->v_;
             this->size_ = len;
             this->v_ = nv;
         }
+    }
     else
     {
         // Or only #ifdef FULLDEBUG