mod_push_keepalive: Don't let hook crash

Check whether mod_push_keepalive is loaded for a given host before
querying the module configuration for that host.  This avoids a hook
crash in the case where the module is enabled for some but not all
hosts.

src/mod_push_keepalive.erl

@@ -234,8 +234,11 @@ c2s_handle_info(State, _) ->
 
 -spec ejabberd_started() -> ok.
 ejabberd_started() ->
-    [wake_all(Host) || Host <- ejabberd_config:get_option(hosts),
-		       mod_push_keepalive_opt:wake_on_start(Host)],
+    Pred = fun(Host) ->
+		   gen_mod:is_loaded(Host, ?MODULE) andalso
+		       mod_push_keepalive_opt:wake_on_start(Host)
+	   end,
+    [wake_all(Host) || Host <- ejabberd_config:get_option(hosts), Pred(Host)],
     ok.
 
 %%--------------------------------------------------------------------