Ignore values of .../mouse_button_modifier key if the key's missing.

2008-10-26  Thomas Thurman  <tthurman@gnome.org>

        * src/core/prefs.c (mouse_button_mods_handler): Ignore values
          of .../mouse_button_modifier key if the key's missing.
          Closes Launchpad bug #258054, Launchpad bug #266929.


svn path=/trunk/; revision=4001