[logging] Don't incorrectly log that REJECT messages are unknown.

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
```

src/net_processing.cpp

@@ -1571,6 +1571,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
                 LogPrint(BCLog::NET, "Unparseable reject message received\n");
             }
         }
+        return true;
     }
 
     else if (strCommand == NetMsgType::VERSION)