fix below zero message count check

size_t is unsigned and hence can't be below zero, triggering this gcc
warning with gcc 10:
  warning: comparison of unsigned expression in ‘< 0’ is always false [-Wtype-limits]

It seems this if statement is meant to check if there are messages to
process (larger than 0). If there are no messages, we should jump out
early.

src/stream.c

@@ -120,7 +120,7 @@ static int stream__flush_plain(struct stream* self)
 			break;
 	}
 
-	if (n_msgs < 0)
+	if (n_msgs == 0)
 		return 0;
 
 	bytes_sent = writev(self->fd, iov, n_msgs);