view.addr(): throw error on bad subnet

It's more consistent with what we do.  Now it will look like:
[system] error while loading config: .../lib/knot-resolver/kres_modules/view.lua:28: failed to parse subnet [::1]/128 (workdir '/foo/bar')

NEWS

+Knot Resolver 5.2.2 (2021-0m-dd)
+================================
+
+Bugfixes
+--------
+- view: fail config if bad subnet is specified (!1112)
+
+
 Knot Resolver 5.2.1 (2020-12-09)
 ================================
 

modules/view/view.lua

@@ -24,6 +24,9 @@ function view.addr(_, subnet, rules, dst)
 	local subnet_cd = ffi.new('char[16]')
 	local family = C.kr_straddr_family(subnet)
 	local bitlen = C.kr_straddr_subnet(subnet_cd, subnet)
+	if bitlen < 0 then
+		error(string.format('failed to parse subnet %s', subnet))
+	end
 	local t = {family, subnet_cd, bitlen, rules}
 	table.insert(dst and view.dst or view.src, t)
 	return t