Fix Tokenizer::int64() parsing of "0" when guessing base (#1842)

Fix Tokenizer::int64() parsing of "0" when guessing base (#1842)

Known bug victims in current code were tcp_outgoing_mark,
mark_client_packet, clientside_mark, and mark_client_connection
directives as well as client_connection_mark and (deprecated)
clientside_mark ACLs if they were configured to match a zero mark using
"0" or "0/..." syntax:

    ERROR: configuration failure: NfMarkConfig: invalid value '0/10'...
    exception location: NfMarkConfig.cc(23) getNfmark

Probably broken since 2014 commit 01f2137d.