Fix: cpp-common/bt2: Pointer comparison in ConstErrorIterator `operator==()`

Fix: cpp-common/bt2: Pointer comparison in ConstErrorIterator `operator==()`

`_mError` and `other._mError` are pointers already. Before this change,
using the iterator would assert at run-time, e.g.

    10-22 15:30:52.152 2434504 2434504 D LIB/CUR-THREAD bt_current_thread_take_error@lib/current-thread.c:36 Took current thread's error object: addr=0x210c19a0

     (╯°□°)╯︵ ┻━┻  ../src/cpp-common/bt2/error.hpp:229: operator==(): Assertion `&other._mError == &_mError` failed.
    Aborted

Signed-off-by: Jérémie Galarneau <jgalar@efficios.com>
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Change-Id: I678b7542e0c94e16e404519a39356079b3719401
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13389
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>