evaluate: check for NULL datatype in rhs in lookup expr

evaluate: check for NULL datatype in rhs in lookup expr

If we are evaluating an EXPR_SET_REF, check if right->dtype is not NULL.
We can hit SEGFAULT if for whatever reason the referenced object does not
exist.

Using this testfile (note the invalid set syntax):

% cat test.nft
flush ruleset
add table t
add chain t c
add set t s {type ipv4_addr\;}
add rule t c ip saddr @s

Without this patch:

% nft -f test.nft
Segmentation fault

With this patch:

% nft -f test.nft
t.nft:4:28-28: Error: syntax error, unexpected junk, expecting newline or semicolon
add set t s {type ipv4_addr\;}
                           ^
t.nft:4:13-29: Error: set definition does not specify key data type
add set t s {type ipv4_addr\;}
            ^^^^^^^^^^^^^^^^^
t.nft:5:23-24: Error: the referenced set does not exist
add rule t c ip saddr @s
             ~~~~~~~~ ^^

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>