networkBandwidthUpdate: Don't blindly dereference pointers
    
    It may happen that an interface don't have any bandwidth set and
    a new one is to be set. In that case, @ifaceBand will be NULL.
    This will cause troubles later in the code when deciding what to
    do.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>