fix send message through interface -1 (NET_INTERFACE_INVALID)

The simulation would crash when trying to access the NIC -1. This invalid interface index was returned from RouteZrpNdp.getMacId(), and this error return code was not checked. It is not clear if this reveals a deeper bug (there always should be a valid NIC for the destination address that triggered the bug -- a bug in the business logic of RouteZrp) or if this was just an unhandled error code. Handling the error code and dropping the message to be sent seems to fix the issue.