tls: code shrink

tls: code shrink

function                                             old     new   delta
tls_get_zeroed_outbuf                                  -      28     +28
static.empty_client_cert                               7       -      -7
tls_handshake                                       1930    1890     -40
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 0/1 up/down: 28/-47)            Total: -19 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>