wiregen: Passing ctx to array helpers that require it

Some of the struct array helpers need to allocate data when
deserializing their fields. The `getnodes` reply is one such example
that allocates the hostname. Since the change to calling array helpers
the getnodes call was broken because it was attempting to allocate off
of the entry, which did not have a tal header, thus failing.

tools/generate-wire.py

@@ -23,6 +23,11 @@ type2size = {
     'bool': 1
 }
 
+# These struct array helpers require a context to allocate from.
+varlen_structs = [
+    'gossip_getnodes_entry',
+]
+
 class FieldType(object):
     def __init__(self,name):
         self.name = name
@@ -235,8 +240,9 @@ class Message(object):
                 subcalls.append('\t\t{}[i] = fromwire_{}(&cursor, plen);'
                                 .format(name, basetype))
             else:
-                subcalls.append('\t\tfromwire_{}(&cursor, plen, {} + i);'
-                                .format(basetype, name))
+                ctx = "ctx, " if basetype in varlen_structs else ""
+                subcalls.append('\t\tfromwire_{}({}&cursor, plen, {} + i);'
+                                .format(basetype, ctx, name))
 
     def print_fromwire(self,is_header):
         ctx_arg = 'const tal_t *ctx, ' if self.has_variable_fields else ''