drm/i915: Prevent dereference of engine before NULL check in error capture

smatch caught,
drivers/gpu/drm/i915/i915_gpu_error.c:1418 gem_record_rings() warn: variable dereferenced before check 'engine' (see line 1413)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621135246.20683-1-chris@chris-wilson.co.uk