when falling back to the default theme, set $chosen_theme to the path

when falling back to the default theme, set $chosen_theme to the path
of default to make functions that depend on $chosen_theme work
correctly (most notably the select list under display preferences which
would have Alien Glow selected when user has no theme set). Thanks Tim Craig
for the bugreport.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@5382 7612ce4b-ef26-0410-bec9-ea0150e637f0