xdgmime: Fix an invalid read

This commit factors out a function for comparing string suffixes, and at
the same time makes it safe for mime types that are shorter than the
"/*" suffix.

==25418== Invalid read of size 1
==25418==    at 0x3C6D0F9D22: __gio_xdg_cache_mime_type_subclass (xdgmimecache.c:848)
==25418==    by 0x3C6D09ED8C: g_content_type_is_a (gcontenttype.c:158)
==25418==    by 0x34D8031E95: gtk_recent_filter_filter (gtkrecentfilter.c:733)
==25418==    by 0x34D802F167: _gtk_recent_chooser_get_items (gtkrecentchooserutils.c:387)
==25418==    by 0x34D802D07F: idle_populate_func (gtkrecentchoosermenu.c:1011)
==25418==    by 0x34D7A20477: gdk_threads_dispatch (gdk.c:804)
==25418==    by 0x3C6C0492F5: g_main_context_dispatch (gmain.c:3065)
==25418==    by 0x3C6C049677: g_main_context_iterate.isra.23 (gmain.c:3712)
==25418==    by 0x3C6C04972B: g_main_context_iteration (gmain.c:3773)
==25418==    by 0x34D7FC2AF4: gtk_main_iteration (gtkmain.c:1262)
==25418==    by 0x408EB4: main (in /usr/bin/glade)

h...