TT#15822 mod_sipwise_pushd: fix error introduced back in 2017

at 23e737f82d we removed invite variable

Change-Id: I28122f9c1dc846bc3a8b0c08ef26e69747b48aa5
(cherry picked from commit e7ecca6910)

plugins/mod_sipwise_pushd.lua

@@ -357,7 +357,7 @@ local function handle_offline(event)
 				origin.host or caller.host);
 			if is_invite(stanza) then
 				msg.data_type = 'invite';
-				caller_jid = jid_bare(invite:get_child('invite').attr.from) or
+				caller_jid = jid_bare(stanza:get_child('invite').attr.from) or
 					caller_jid;
 			elseif is_attachment(stanza) then
 				msg.data_message = "new attachment";