src: fix 'describe' command when passing wrong expressions

src: fix 'describe' command when passing wrong expressions

Before this patch:

 # nft describe tcp foo
 value expression, datatype inet_proto (Internet protocol) (basetype integer), 8 bits
 Segmentation fault

After this patch:

 # nft describe tcp foo
 <cmdline>:1:14-16: Error: syntax error, unexpected string, expecting end of file or newline or semicolon
 describe tcp foo
              ^^^

Reported-by: Kevin Fenzi <kevin@scrye.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>