RadioLibInterface: clear static instance on destruction to prevent UAF (#10254)

The constructor sets `RadioLibInterface::instance = this` immediately,
before `init()` runs. `initLoRa()` in RadioInterface.cpp creates each
radio variant with `new SX1262Interface(...)` or similar, then calls
`init()`, and if init fails the `unique_ptr<RadioInterface>` is reset
to nullptr — destroying the object — while the static `instance`
pointer continues to point at the freed memory.

Main loop then checks `RadioLibInterface::instance != nullptr` and
calls `pollMissedIrqs()` or `resetAGC()` on the dangling pointer →
Guru Meditation (IllegalInstruction / LoadProhibited).

Reported in #9880 on an ESP32-S3 dev board without radio hardware
attached, where init always fails and the leftover pointer crashes
the device on the next `loop()` iteration.

Fix: add a virtual destructor to `RadioLibInterface` that clears the
static pointer iff it still references this object. A later successful
init() may have replaced `instance` with a different interface — the
`instance == this` guard preserves that case.

Fixes #9880

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/RadioLibInterface.h

@@ -136,6 +136,13 @@ class RadioLibInterface : public RadioInterface, protected concurrency::Notified
     RadioLibInterface(LockingArduinoHal *hal, RADIOLIB_PIN_TYPE cs, RADIOLIB_PIN_TYPE irq, RADIOLIB_PIN_TYPE rst,
                       RADIOLIB_PIN_TYPE busy, PhysicalLayer *iface = NULL);
 
+    /**
+     * Clear the static `instance` pointer if it still points at us, so callers
+     * that check `RadioLibInterface::instance != nullptr` don't dereference a
+     * freed object after a failed init() + unique_ptr reset.
+     */
+    virtual ~RadioLibInterface();
+
     virtual ErrorCode send(meshtastic_MeshPacket *p) override;
 
     /**