libcli:nbt: Initialize struct nbt_name_register

libcli:nbt: Initialize struct nbt_name_register

"Error: UNINIT (CWE-457):
samba-4.20.0rc2/libcli/nbt/pynbt.c:277: var_decl: Declaring variable ""io"" without initializer.
samba-4.20.0rc2/libcli/nbt/pynbt.c:305: uninit_use_in_call: Using uninitialized value ""io.in.nb_flags"" when calling ""nbt_name_register"".
  303|   		return NULL;
  304|
  305|-> 	status = nbt_name_register(node->socket, NULL, &io);
  306|
  307|   	if (NT_STATUS_IS_ERR(status)) {"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>