Discovery based on agent item fails

Discoverer memset()'s DC_ITEM structure with zeros and passes it to get_value_agent() where item->tls_connect == 0 is interpreted not ZBX_TCP_SEC_UNENCRYPTED.

Obviously, nothing good will happen if encryption support was compiled in when zbx_tcp_connect() gets tls_connect == 0, tls_arg1 == NULL and tls_arg2 == NULL.

If encryption support was not compiled in get_value_agent() will fail with error: "A TLS connection is configured to be used with agent but support for TLS was not compiled into %s."

Also, in this scenario zbx_tcp_close() is called for uninitialized socket.

Introduced with ZBXNEXT-1263.