fix: improve memory safety and UART driver stability

[H3rD3m1s3MyR1s3]

PR Description:

 This PR addresses several stability issues and potential crash points in t
  ESP32 control module, specifically focusing on memory management and
  hardware driver lifecycle.

 ### Key Changes:

 1. **Memory Safety (Heap Exhaustion Protection):**
    - Added `NULL` checks for `malloc()` calls in `db_send_to_all_espnow` a
      the Bluetooth (BLE) transmission path.
    - **Benefit:** Prevents Kernel Panics and random reboots if the system
      runs low on memory during high MAVLink throughput. The system will now log
      an error and skip the packet instead of crashing.

 2. **UART Driver Management:**
   - Added a `uart_is_driver_installed()` check before calling
    `uart_driver_install()` in `db_serial.c`.
   - **Benefit:** Prevents resource leaks and driver initialization errors
    when the control module is restarted (e.g., after a settings change in the
    Web UI).

3. Buffer Protection:

   • Implemented a missing bounds check in handle_internal_telemetry.
   • Benefit: Prevents memory corruption/over-reads if a malformed or
     truncated internal telemetry packet is received from another station.

4. Hardware Efficiency:

   • Added a zero-length guard in write_to_serial().
   • Benefit: Prevents unnecessary calls to the underlying hardware
     drivers when there is no data to transmit.

### Testing Performed:
    - Verified stable UART re-initialization after settings changes.
    - Verified system stability under high data load (parameter downloads in
      QGroundControl).

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