BUG: potential memory leaks in HashPtrTable (#1787)

- using HashPtrTable::set() with the same key twice did not guarantee
  proper cleanup of memory since it simply used the underlying
  HashTable::set() without doing anything about the old memory. Now
  check for pre-existing storage and delete it when it does not
  correspond to the newly stored pointer.

  This problem is independent of potential memory slicing previously
  flagged (#1286) and only partially resolved.

src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H

@@ -28,7 +28,8 @@ Class
     Foam::HashPtrTable
 
 Description
-    A HashTable of pointers to objects of type \<T\>.
+    A HashTable of pointers to objects of type \<T\>,
+    with deallocation management of the pointers.
 
 SourceFiles
     HashPtrTable.C

src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableI.H

@@ -144,7 +144,16 @@ inline bool Foam::HashPtrTable<T, Key, Hash>::set
     T* ptr
 )
 {
-    return this->parent_type::set(key, ptr);
+    const T* old = this->get(key);
+
+    const bool ok = this->parent_type::set(key, ptr);
+
+    if (ok && old != ptr)
+    {
+        delete const_cast<T*>(old);
+    }
+
+    return ok;
 }