Fix spin-loop in websocket write when socket buffer is full

If the socket buffer is (almost) full and can hold some of the websocket
frame we made, a return value of 0 bytes written would make it try
forever (calling the write() syscall), until either the socket buffer
was drained, or until it errored out with ETIMEDOUT.

iowrapper.cpp

@@ -999,7 +999,7 @@ ssize_t IoWrapper::writeWebsocketAndOrSsl(int fd, const void *buf, size_t nbytes
 
             if (n > 0)
                 websocketWriteRemainder.advanceTail(n);
-            if (n < 0)
+            else
                 break;
         }