datatype: fix crash if wrong integer type is passed

datatype: fix crash if wrong integer type is passed

Eric Leblond reported that this command:

nft add rule ip6 filter input position 4 meta protocol icmpv6 accept

crashes nft. The problem is that 'icmpv6' is wrong there, as
meta protocol is expecting an ethernet protocol, that can be
expressed as an hexadecimal.

Now this command displays the following error:

<cmdline>:1:52-57: Error: This is not a valid Ethernet protocol
add rule ip6 filter input position 4 meta protocol icmpv6 accept
                                                   ^^^^^^

This closes bugzilla #834:
https://bugzilla.netfilter.org/show_bug.cgi?id=834

Reported-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>