expr: catch missing and excess elements in concatenations

expr: catch missing and excess elements in concatenations

# nft -nn filter output ip daddr . tcp dport . tcp dport { 192.168.0.1 . ssh }
<cmdline>:1:50-66: Error: datatype mismatch, expected concatenation of (IPv4 address, internet network service, internet network service), expression has type concatenation of (IPv4 address, internet network service)
filter output ip daddr . tcp dport . tcp dport { 192.168.0.1 . ssh }
                                                 ^^^^^^^^^^^^^^^^^

# nft -nn filter output ip daddr . tcp dport . tcp dport { 192.168.0.1 . ssh . ssh . ssh}
<cmdline>:1:76-78: Error: unexpected concat component, expecting concatenation of (IPv4 address, internet network service, internet network service)
filter output ip daddr . tcp dport . tcp dport { 192.168.0.1 . ssh . ssh . ssh}
                                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^

Signed-off-by: Patrick McHardy <kaber@trash.net>