netlink: don't crash when set elements are not evaluated as expected

netlink: don't crash when set elements are not evaluated as expected

define foo = 2001:db8:123::/48

table inet filter {
	set foo {
		typeof ip6 saddr
		elements = $foo
	}
}

gives crash.  This now exits with:

stdin:1:14-30: Error: Unexpected initial set type prefix
define foo = 2001:db8:123::/48
             ^^^^^^^^^^^^^^^^^

For literals, bison parser protects us, as it enforces
'elements = { 2001:... '.

For 'elements = $foo' we can't detect it at parsing stage as the '$foo'
symbol might as well evaluate to "{ 2001, ...}" (i.e. we can't do a
set element allocation).

So at least detect this from set instantiaton.

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