Fix BluetoothSerial memory leak

[rafern]

An easy way to trigger this leak is to only send packets with a size multiple and greater than SPP_TX_MAX (i.e. 660 bytes of data). This example will easily make the device run out of memory:

#include "BluetoothSerial.h"
BluetoothSerial SerialBT;

void setup() {
  Serial.begin(115200);
  SerialBT.begin("ESP32test");
}

void loop() {
  log_i("Free heap: %zu", ESP.getFreeHeap());
  SerialBT.write((uint8_t*)"Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cil.Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cil..Duis aute irure dolor in reprehenderit in voluptate velit ..", 660);
  delay(20);
}

Note that you must connect a device via bluetooth to the esp for the messages to start sending, else, this program does nothing as no messages are being sent

[me-no-dev]

good catch :)