Fix tls1_set_sigalgs() length calculation

The length passed to tls1_set_sigalgs() is a multiple of two and there are
two char entries in the list for each sigalg. When we set
client_sigalgslen or conf_sigalgslen this is the number of ints in the list
where there is one entry per sigalg (i.e. half the length of the list passed
to the function).

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2157)