sqimap_utf7_decode_mbx_tree doesn't issue a return, but simply changes

sqimap_utf7_decode_mbx_tree doesn't issue a return, but simply changes
the variables by reference to the call.  This makes mbxs_tree an undefined
and unusued variable and might cause errors in php 4.4+ (especially with
objects).

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@10309 7612ce4b-ef26-0410-bec9-ea0150e637f0