Don't store escaped serial characters if comment_mode

Marlin/Marlin_main.cpp

       if (MYSERIAL.available() > 0) {
         // if we have one more character, copy it over
         serial_char = MYSERIAL.read();
-        serial_line_buffer[serial_count++] = serial_char;
+        if (!serial_comment_mode) serial_line_buffer[serial_count++] = serial_char;
       }
       // otherwise do nothing
     }