Try to fix some gwarnings when pidgin_blist_update_sort_method gets called before the menu of the buddy list exists. This is pretty crappy, but this just helps me get through gdb faster right now.

There are some conditions where pidgin_blist_update_sort_methods will get called before the menu widget exists.

Testing Done:
Compiled and ran in gdb with `G_DEBUG=fatal-warnings`.

Reviewed at https://reviews.imfreedom.org/r/101/