drm/i915/selftests: Initialise err in case there are no engines!

drivers/gpu/drm/i915//gt/selftest_engine_heartbeat.c:255 live_heartbeat_fast() error: uninitialized symbol 'err'.
drivers/gpu/drm/i915//gt/selftest_engine_heartbeat.c:320 live_heartbeat_off() error: uninitialized symbol 'err'.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025135943.12524-2-chris@chris-wilson.co.uk