drm/i915: Nul-terminate legacy debug string

Make sure that when we don't have any scheduler attributes for the
request, the string is terminated.

Fixes: 247870ac8ea7 ("drm/i915: Build request info on stack before printk")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517152824.11619-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1143,7 +1143,7 @@ static void print_request(struct drm_printer *m,
 			  const char *prefix)
 {
 	const char *name = rq->fence.ops->get_timeline_name(&rq->fence);
-	char buf[80];
+	char buf[80] = "";
 	int x = 0;
 
 	x = print_sched_attr(rq->i915, &rq->sched.attr, buf, x, sizeof(buf));