Reject messages are logged to debug.log if NET debug logging is enabled.

Because of the way the `ProcessMessages()` function is structured,
processing for REJECT messages will also drop through to the default
branch and incorrectly log `Unknown command "reject" from peer-?`. Fix
that by exiting from `ProcessMessages()` early.

without this PR:
```
2018-05-03T17:37:00.930600Z received: reject (21 bytes) peer=0
2018-05-03T17:37:00.930620Z Reject message code 16: spammy spam
2018-05-03T17:37:00.930656Z Unknown command "reject" from peer=0
```
with this PR:
```
2018-05-03T17:35:04.751246Z received: reject (21 bytes) peer=0
2018-05-03T17:35:04.751274Z Reject message code 16: spammy spam
```