tpm-emulator: fix error handling

The previous patch cleaned up a bit error handling, and exposed an
existing bug: error_report_err() could be called with a NULL error.
Instead, make tpm_emulator_set_locality() set the error.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
