client: fix icon value on changes

Sometimes we unref the icon image, but we did not reset NULL, which
can make the client refering to a not-refed image.

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

client.c

@@ -1521,6 +1521,7 @@ luaA_client_newindex(lua_State *L)
         break;
       case A_TK_ICON:
         image_unref(L, c->icon);
+        c->icon = NULL;
         c->icon = image_ref(L);
         /* execute hook */
         hooks_property(c, "icon");

property.c

@@ -354,6 +354,8 @@ property_handle_net_wm_icon(void *data,
         image_unref(globalconf.L, c->icon);
         if(ewmh_window_icon_from_reply(reply))
             c->icon = image_ref(globalconf.L);
+        else
+            c->icon = NULL;
         /* execute hook */
         hooks_property(c, "icon");
     }