Ticket 47988: Schema learning mechanism, in replication, unable to extend an existing definition
    
    Description: Covscan FORWARD_NULL fix introduced by commit
    commit 51e05df9c37c66206041f026c9a67ec17bc9ea4a
    
    1. ldap/servers/slapd/schema.c:2490: var_deref_op: Dereferencing null pointer "mod->mod_vals.modv_bvals".
    2. ldap/servers/slapd/schema.c:2603: var_deref_op: Dereferencing null pointer "mod->mod_vals.modv_bvals".
    
    Added NULL check on mod->mod_bvalues to the for loop stop condition.