Bug 679978 - modifying attr value crashes the server, which is supposed to
     be indexed as substring type, but has octetstring syntax
    
    https://bugzilla.redhat.com/show_bug.cgi?id=679978
    
    Description: When indexing, index_addordel_values_ext_sv calls a helper
    function valuearray_minus_valuearray in index.c.  There is a corner case
    that could pass NULL array arguments to valuearray_minus_valuearray.
    In valuearray_minus_valuearray, the array's elements were accessed w/o
    checking the array was NULL or not.  This patch is adding the check.