util/error: Fix use-after-free errors reported by Coverity

Fix use-after-free errors in the code path that called error_handle(). A
call to error_handle() will now either free the passed Error 'err' or
assign it to '*errp' if '*errp' is currently NULL. This ensures that 'err'
either has been freed or is assigned to '*errp' if this function returns.
Adjust the two callers of this function to not assign the 'err' to '*errp'
themselves, since this is now handled by error_handle().

Fixes: commit 3ffef1a5 ("error: add global &error_warn destination")
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20230406154347.4100700-1-stefanb@linux.ibm.com