evaluate: reset ctx->set after set interval evaluation

evaluate: reset ctx->set after set interval evaluation

Otherwise bogus error reports on set datatype mismatch might occur, such as:

Error: datatype mismatch, expected Internet protocol, expression has type IPv4 address
    meta l4proto { tcp, udp } th dport 443 dnat to 10.0.0.1
    ~~~~~~~~~~~~ ^^^^^^^^^^^^

with an unrelated set declaration.

table ip test {
       set set_with_interval {
               type ipv4_addr
               flags interval
       }

       chain prerouting {
               type nat hook prerouting priority dstnat; policy accept;
               meta l4proto { tcp, udp } th dport 443 dnat to 10.0.0.1
       }
}

This bug has been introduced in the evaluation step.

Reported-by: Roman Petrov <nwhisper@gmail.com>
Fixes: 81e36530fcac ("src: replace interval segment tree overlap and automerge)"
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>