The problem is caused by gtk_im_multicontext_set_slave(), which forgets
to disconnect these signal handlers:
* gtk_im_multicontext_retrieve_surrounding_cb
* gtk_im_multicontext_delete_surrounding_cb

If slave GtkImContext emits signal after GtkIMMulticontext context is
destroyed, this leads to reading freed memory, sometimes causing a crash.

Fixes: #2365