BUG/MINOR: ssl-hello: make use of the null-terminated servername

BUG/MINOR: ssl-hello: make use of the null-terminated servername

In ssl_sock_switchctx_cbk(), the servername is copied into the trash
and null-terminated, but later in the call to strncpy() it's still used
as-is, so anything that follows it will be copied as well, which is not
really expected. Let's make the servername point to the trash after
sanitizing it, like ssl_sock_switchcbk_wolfSSL_cbk() does.

This can be backported to 2.6 since it was introduced with commit
a996763619 ("BUG/MINOR: ssl: Store client SNI in SSL context in case
of ClientHello error").