[tls] Ensure received data list is initialised before calling tls_free()

[tls] Ensure received data list is initialised before calling tls_free()

A failure in tls_generate_random() will result in a call to ref_put()
before the received data list has been initialised, which will cause
free_tls() to attempt to traverse an uninitialised list.

Fix by ensuring that all fields referenced by free_tls() are
initialised before any of the potential failure paths.

Signed-off-by: Michael Brown <mcb30@ipxe.org>