Fix: set uip_ext_len to 0 after dropping packet

###### Bug description:
If outgoing packet with hop-by-hop header is dropped in `tcpip_ipv6_output()` function, uip_ext_len is not set to 0. It causes next packet with no hop-by-hop header (for example ICMP) to become corrupt. Bug can be easily observed with ipv6/rpl-udp example client. If client looses connection with parrent, some outgoing DIO messages become corrupt (depending on the frequency of outgoing UDP messages). 

###### Fix description:
Issue can be solved by adding:
```
uip_ext_len = 0;
```
before every return statement in `tcpip_ipv6_output()`. 

After comment on mailing list from Laurent Deru, `uip_ext_len = 0` is also added in other places where packet is discarded.