fix uart out of bounds warning

src/uart/emsuart_esp32.cpp

@@ -67,9 +67,11 @@ void IRAM_ATTR EMSuart::emsuart_rx_intr_handler(void * para) {
                 drop_next_rx_ = true; // we have a overflow
             }
         }
+        if (length > 0 && length < EMS_MAXBUFFERSIZE) {
             if (rxbuf[length - 1]) { // check if last byte is break
                 length++;
             }
+        }
         if ((!drop_next_rx_) && ((length == 2) || (length > 4))) {
             int baseType = 0;
             xRingbufferSendFromISR(buf_handle_, rxbuf, length - 1, &baseType);