Ticket 328 - make sure all internal search filters are properly escaped
    
    Fix description:
    Fixing double-free issues introduced with commit 3cf9a521fa43183c657a5dca507dec3a4379e383.
    1) If ctx.buf is realloc'ed in filter_stuff_func, slapi_filter_sprintf
       returns already freed buf.  And the buf is freed by caller, again.
    2) If filter escape occurs in get_substring_filter, freed val is
       returned to caller via "struct slapi_filter *f" and it is freed
       again.