protect unconnected USB-CDC from being used

found a few more places where Serial was used without first checking if its connected.

Arduino docs: `if (Serial)` indicates whether or not the USB CDC serial connection is open. For all non-USB CDC ports, this will always return true

tools/ESP32-Chip_info.hpp

@@ -543,6 +543,8 @@ void show_psram_info_part2(void)
 
 void showRealSpeed() {
   //Serial.begin(115200);
+  if (!Serial) return; // Avoid writing to unconnected USB-CDC
+
   Serial.flush();
   Serial.println(F("\n"));
   for(int aa=0; aa<65; aa++) Serial.print("="); Serial.println();