intervals: BUG on prefix expressions without value

intervals: BUG on prefix expressions without value

Its possible to end up with prefix expressions that have
a symbolic expression, e.g.:

table t {
        set s {
                type inet_service
                flags interval
                elements = { 172.16.0.0/16 }
        }

        set s {
                type inet_service
                flags interval
                elements = { 0-1024, 8080-8082, 10000-40000 }
        }
}

Without this change, nft will crash.  We end up in setelem_expr_to_range()
with prefix "/16" for the symbolic expression "172.16.0.0".

We than pass invalid mpz_t pointer into libgmp.

This isn't a real fix, but instead of blindly assuming that the attached
expression has a gmp value die with at least some info.

Signed-off-by: Florian Westphal <fw@strlen.de>