clear the pal gap queue on reset

connectivity/FEATURE_BLE/source/generic/GapImpl.cpp

@@ -1214,6 +1214,7 @@ ble_error_t Gap::reset()
     /* Notify that the instance is about to shut down */
 //    shutdownCallChain.call(this);
     shutdownCallChain.clear();
+    _event_queue.clear();
 
     _event_handler = nullptr;
     _initiating = false;

connectivity/FEATURE_BLE/source/pal/PalEventQueue.h

@@ -52,6 +52,10 @@ public:
      * BLEInstanceBase::process
      */
     virtual bool post(const mbed::Callback<void()>& event) = 0;
+
+    /** Remove all pending events.
+     */
+    virtual void clear() = 0;
 };
 
 } // namespace ble