Merge #13162: [net] Don't incorrectly log that REJECT messages are unknown.

fad63eb [logging] Don't incorrectly log that REJECT messages are unknown. (John Newbery)

Pull request description:

  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
  ```

Tree-SHA512: 5c84c98433ab99e0db2dd481f9c2db6f87ff0d39022ff317a791737e918714bbcb4a23e81118212ed8e594ebcf098ab7f52f7fd5e21ebc3f07b1efb279b9b30b

Author: laanwj

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)