drm/i915/selftest/buddy: fixup igt_buddy_alloc_range

Dan reported the following static checker warning:

drivers/gpu/drm/i915/selftests/i915_buddy.c:670 igt_buddy_alloc_range()
error: we previously assumed 'block' could be null (see line 665)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190815103210.11802-1-matthew.auld@intel.com