tools/generate-wire.py: fix loop logic for towire_xxx_array

has_len_fields() doesn't cover our blacklist of variable types, so if
we have an array of them, this logic is wrong.  This happens in the
the bolt13 patch:

```C
struct tlv_offer_tlvs_blindedpath {
        struct pubkey blinding;
        struct onionmsg_path **path;
};
```

Before:
wire/gen_bolt13_tlv.c:
```C
		for (size_t i = 0; i < tal_count(r->blindedpath->path); i++)
		towire_onionmsg_path(&ptr, r->blindedpath->path + i);
```
After:
```C
		for (size_t i = 0; i < tal_count(r->blindedpath->path); i++)
		towire_onionmsg_path(&ptr, r->blindedpath->path[i]);
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/gen/impl_template

@@ -53,7 +53,7 @@ bool ${enum_set['name']}_is_defined(u16 type)
 towire_${type_obj.name}_array(${ptr}, ${fieldname}, ${f.size('tal_count(' + fieldname + ')')});
     % else:
 	for (size_t i = 0; i < ${f.size('tal_count(' + fieldname + ')')}; i++)
-	% if type_obj.is_assignable() or type_obj.has_len_fields():
+	% if type_obj.is_assignable() or type_obj.is_varsize():
 		towire_${type_obj.name}(${ptr}, ${fieldname}[i]);
 	% else:
 		towire_${type_obj.name}(${ptr}, ${fieldname} + i);