Don't use OPENSSL_strdup() for copying alpn_selected

An alpn_selected value containing NUL bytes in it will result in
ext.alpn_selected_len having a larger value than the number of bytes
allocated in ext.alpn_selected.

Issue found by OSS-fuzz.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6507)