selftests: tls: fix warning of uninitialized variable

selftests: tls: fix warning of uninitialized variable

In 'poll_partial_rec_async' a uninitialized char variable 'token' with
is used for write/read instruction to synchronize between threads
via a pipe.

tls.c:2833:26: warning: variable 'token' is uninitialized
      		   when passed as a const pointer argument

Initialize 'token' to '\0' to silence compiler warning.

Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Link: https://patch.msgid.link/20251205163242.14615-1-ankitkhushwaha.linux@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>