Clear alpn_selected_len for clients, too

Zero out the length alongside the NULLing of the pointer, to
bring parity between the selected and proposed fields..

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2954)