[lua] Fix garbage collectors

Signed-off-by: Julien Danjou <julien@danjou.info>

keybinding.c

@@ -127,6 +127,7 @@ luaA_keybinding_gc(lua_State *L)
 {
     keybinding_t **keybinding = luaL_checkudata(L, 1, "keybinding");
     keybinding_unref(keybinding);
+    keybinding = NULL;
     return 0;
 }
 

statusbar.c

@@ -503,6 +503,7 @@ luaA_statusbar_gc(lua_State *L)
 {
     statusbar_t **sb = luaL_checkudata(L, 1, "statusbar");
     statusbar_unref(sb);
+    *sb = NULL;
     return 0;
 }
 

tag.c

@@ -484,6 +484,7 @@ luaA_tag_gc(lua_State *L)
 {
     tag_t **tag = luaL_checkudata(L, 1, "tag");
     tag_unref(tag);
+    *tag = NULL;
     return 0;
 }
 

titlebar.c

@@ -527,6 +527,7 @@ luaA_titlebar_gc(lua_State *L)
 {
     titlebar_t **titlebar = luaL_checkudata(L, 1, "titlebar");
     titlebar_unref(titlebar);
+    *titlebar = NULL;
     return 0;
 }
 

widget.c

@@ -438,6 +438,7 @@ luaA_widget_gc(lua_State *L)
 {
     widget_t **widget = luaL_checkudata(L, 1, "widget");
     widget_unref(widget);
+    *widget = NULL;
     return 0;
 }