BUG/MINOR: ssl: reject a \0 byte in ssl_fc_npn

BUG/MINOR: ssl: reject a \0 byte in ssl_fc_npn

A string sample cannot contain a \0 byte, but ssl_fc_npn returned the
NPN protocol name as sent by the client, which OpenSSL stores without
checking it against the configured list, so a client could put a \0 in
it. Such a value was then matched by "-m str" on its prefix.

Fail the fetch when the name holds a \0 byte. This only applies when
"npn" is set on the bind line.

This must be backported to all stable branches.