virnetdev: Resolve Coverity FORWARD_NULL
    
    The complaint is that if cleanup is called when virFileReadAll fails,
    then mcast->entries is NULL and could be dereferenced in the clear
    function. After following the code some - I saw that the caller to
    the function (virNetDevGetMulticastTable) will also call
    virNetDevMcastListClear if this function returns -1, so this
    isn't necessary, so I removed the call.