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language: don't crash if sort_key is NULL

Will Thompson requested to merge 85-gnome-intial-setup-segfaults-possibly-due-to-locale-related-problems into master

On #85 (closed) we see the following backtrace, where the process has segfaulted:

#0  0x00007f40bd849c02 in __strcmp_avx2 () at /usr/lib/libc.so.6
#1  0x00007f40bd2c77cc in node_find_closest
    (haystack=0x563cd8093760, needle=needle@entry=0x563cd8093430, end=0x563cd7ef4590, iter_cmp=iter_cmp@entry=0x7f40bd2c7890 <iter_compare>, cmp_data=cmp_data@entry=0x7ffcbeb8bd10)
    at ../glib/glib/gsequence.c:1793
#2  0x00007f40bd2c9f5f in node_insert_sorted
    (node=<optimized out>, new=new@entry=0x563cd8093430, end=<optimized out>, iter_cmp=iter_cmp@entry=0x7f40bd2c7890 <iter_compare>, cmp_data=cmp_data@entry=0x7ffcbeb8bd10) at ../glib/glib/gsequence.c:2031
#3  0x00007f40bd2cad96 in g_sequence_insert_sorted_iter (seq=0x563cd81225b0, data=<optimized out>, iter_cmp=iter_cmp@entry=0x7f40bd2c7890 <iter_compare>, cmp_data=cmp_data@entry=0x7ffcbeb8bd10)
    at ../glib/glib/gsequence.c:1047
#4  0x00007f40bd2cae6d in g_sequence_insert_sorted (seq=<optimized out>, data=<optimized out>, cmp_func=<optimized out>, cmp_data=<optimized out>) at ../glib/glib/gsequence.c:740
#5  0x00007f40bcd30b75 in gtk_list_box_insert () at /usr/lib/libgtk-3.so.0
#6  0x00007f40bd3d3881 in g_cclosure_marshal_VOID__OBJECTv
    (closure=0x563cd7940f80, return_value=<optimized out>, instance=<optimized out>, args=<optimized out>, marshal_data=<optimized out>, n_params=<optimized out>, param_types=0x563cd7944b70)
    at ../glib/gobject/gmarshal.c:1910
#7  0x00007f40bd3c3bba in _g_closure_invoke_va (param_types=0x563cd7944b70, n_params=<optimized out>, args=0x7ffcbeb8bf50, instance=0x563cd8124370, return_value=0x0, closure=0x563cd7940f80)
    at ../glib/gobject/gclosure.c:873
#8  0x00007f40bd3c3bba in g_signal_emit_valist (instance=0x563cd8124370, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7ffcbeb8bf50) at ../glib/gobject/gsignal.c:3306
#9  0x00007f40bd3c4868 in g_signal_emit (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>) at ../glib/gobject/gsignal.c:3453
#10 0x00007f40bcdf90ee in gtk_container_add () at /usr/lib/libgtk-3.so.0
#11 0x0000563cd59f02e6 in add_one_language (chooser=chooser@entry=0x563cd7f50c40, locale_id=<optimized out>, is_initial=is_initial@entry=1)
    at ../gnome-initial-setup-3.34.0/gnome-initial-setup/pages/language/cc-language-chooser.c:258

I believe the call to strcmp() here is from sort_languages(), since this is the sort_func for the list box in the language chooser widget; it's a tail call so I think the sort_languages() stack frame has been optimized away.

LanguageWidget.sort_key is defined as:

widget->sort_key = cc_util_normalize_casefold_and_unaccent (locale_name);

locale_name is defined as:

locale_name = gnome_get_language_from_locale (locale_id, locale_id);

gnome_get_language_from_locale() may return NULL in various situations. In this particular case, it may be because the user has not installed various locales, or perhaps the iso-codes package (which provides the human-readable names that libgnome-desktop uses) is not installed.

Replace the strcmp() call with g_strcmp0(), which can tolerate NULLs. Since we want to impose a total order on the list of languages, fall back to comparing locale_id. There's an argument that (a sort key derived from) locale_current_name or locale_untranslated_name – which are the name of the locale in the current locale and in the C locale respectively – should be considered first, but using the locale_id as a last resort seems fine anyway.

Closes #85 (closed)

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