gtkimcontexttime.c: Check context_ime->client_window is not NULL
gtkimcontexttime.c: Check context_ime->client_window is not NULL

Be a bit more careful in get_pango_attr_list() and
get_utf8_preedit_string() to ensure that the client_window is properly
created before proceeding, to avoid access violation/segfault crashes on
Windows with IME installed, especially when running the pickers demo.

https://bugzilla.gnome.org/show_bug.cgi?id=682919
(cherry picked from commit a866ed737810ea32ec55260ad512f386a4e6df5d)

Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk