os: minor fix terminal_expect_send test-component

When an empty line was send, it could occur
that a previous line got printed instead.

Ref #3926

repos/os/src/test/terminal_expect_send/main.cc

@@ -55,6 +55,7 @@ struct Main
 			if (read_buffer[i] == '\n') {
 				if (verbose) log(Cstring(line));
 				line_idx = 0;
+				line[line_idx] = 0;
 			}
 		}
 	}