free tile_thread task_idx_to_sby_and_tile_idx

Fixes emmory leak with asan seen with 'dav1d --tilethreads 2 ...'
7 jobs for master in 1 minute and 58 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build-debian #3458
debian amd64

00:00:28

passed build-debian-aarch64 #3462
aarch64 debian

00:01:28

passed build-debian-aarch64-clang-5 #3463
aarch64 clang5 debian

00:00:56

passed build-debian-static #3459
debian amd64

00:00:28

passed build-macos #3464
macos

00:00:23

passed build-win32 #3460
win32

00:00:27

passed build-win64 #3461
win64

00:00:30