Fix memory leak due to recent port to std::unique_ptr

commit 8eec3450 replaced delete d_ptr
with a std::unique_ptr, but there's a catch in this class hierarchy,
as indicated by the comment about BasePrivate: its destructor isn't
virtual (on purpose) so one needs to delete a pointer with the right
derived type.

Detected by ASAN on bin/utiltest (and probably all others)