dns_resolver: Null-terminate the right string

*_result[len] is parsed as *(_result[len]) which is not at all what we
want to touch here.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: 84a7c0b1

 ("dns_resolver: assure that dns_query() result is null-terminated")
Signed-off-by: David S. Miller <davem@davemloft.net>