Fix infinite loop if thumbnailer is not available
The code in cheese_thumb_view_idle_append_item() in
src/thumbview/cheese-thumb-view.c didn't pop the list of items to
thumbnail if thumbnailing failed.
#0 0x00007f4a60e55314 in open64 () at /lib64/libc.so.6
#1 0x00007f4a60de6386 in _IO_file_open () at /lib64/libc.so.6
#2 0x00007f4a60de655a in __GI__IO_file_fopen () at /lib64/libc.so.6
#3 0x00007f4a60dd9aad in __fopen_internal () at /lib64/libc.so.6
#4 0x00007f4a6157a43f in gdk_pixbuf_new_from_file () at /lib64/libgdk_pixbuf-2.0.so.0
#5 0x00007f4a61e84b3a in gnome_desktop_thumbnail_factory_lookup () at /lib64/libgnome-desktop-3.so.19
#6 0x000055cef476046f in cheese_thumb_view_idle_append_item ()
#7 0x00007f4a6124f47b in g_idle_dispatch () at /lib64/libglib-2.0.so.0
#8 0x00007f4a612537af in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
#9 0x00007f4a61253b38 in g_main_context_iterate.constprop () at /lib64/libglib-2.0.so.0
#10 0x00007f4a61253c03 in g_main_context_iteration () at /lib64/libglib-2.0.so.0
#11 0x00007f4a6146a7ca in g_application_run () at /lib64/libgio-2.0.so.0
#12 0x000055cef4758547 in _vala_main ()
#13 0x00007f4a60d8a042 in __libc_start_main () at /lib64/libc.so.6
#14 0x000055cef47554be in _start ()
#0 0x00007f4a60ec562d in __strlen_avx2 () at /lib64/libc.so.6
#1 0x00007f4a61275de8 in g_str_has_suffix () at /lib64/libglib-2.0.so.0
#2 0x00007f4a618c0072 in icon_name_is_symbolic () at /lib64/libgtk-3.so.0
#3 0x00007f4a618c00b1 in theme_dir_get_icon_suffix () at /lib64/libgtk-3.so.0
#4 0x00007f4a618c32ed in theme_lookup_icon () at /lib64/libgtk-3.so.0
#5 0x00007f4a618c3adf in real_choose_icon () at /lib64/libgtk-3.so.0
#6 0x00007f4a618c4762 in gtk_icon_theme_lookup_icon_for_scale () at /lib64/libgtk-3.so.0
#7 0x00007f4a618c5105 in gtk_icon_theme_load_icon_for_scale () at /lib64/libgtk-3.so.0
#8 0x000055cef47605c9 in cheese_thumb_view_idle_append_item ()
#9 0x00007f4a6124f47b in g_idle_dispatch () at /lib64/libglib-2.0.so.0
#10 0x00007f4a612537af in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
#11 0x00007f4a61253b38 in g_main_context_iterate.constprop () at /lib64/libglib-2.0.so.0
#12 0x00007f4a61253c03 in g_main_context_iteration () at /lib64/libglib-2.0.so.0
#13 0x00007f4a6146a7d8 in g_application_run () at /lib64/libgio-2.0.so.0
#14 0x000055cef4758547 in _vala_main ()
#15 0x00007f4a60d8a042 in __libc_start_main () at /lib64/libc.so.6
#16 0x000055cef47554be in _start ()Closes: #81 (closed)