Initialize detectedBaudRate to prevent compilation errors (#2101)

cores/esp32/HardwareSerial.cpp

@@ -56,7 +56,7 @@ void HardwareSerial::begin(unsigned long baud, uint32_t config, int8_t rxPin, in
 
     if(!baud) {
         time_t startMillis = millis();
-        unsigned long detectedBaudRate;
+        unsigned long detectedBaudRate = 0;
         while(millis() - startMillis < timeout_ms && !(detectedBaudRate = uartDetectBaudrate(_uart))) {
             yield();
         }