Silence smatch over

drivers/gpu/drm/i915/selftests/intel_guc.c:135 igt_guc_init_doorbell_hw() error: we previously assumed 'guc->execbuf_client' could be null (see line 123)
drivers/gpu/drm/i915/selftests/intel_guc.c:142 igt_guc_init_doorbell_hw() error: we previously assumed 'guc->preempt_client' could be null (see line 123)

by asserting that we did succeed in creating the pair of clients for
testing.

References: 55bd6bd7 ("drm/i915/selftests: Add a GuC doorbells selftest")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171120211907.1649-1-chris@chris-wilson.co.ukReviewed-by: Michel Thierry <michel.thierry@intel.com>