Prevent LibGpiodV2 native abort on null edge event

src/System.Device.Gpio/Interop/Unix/libgpiod/V2/Proxies/EdgeEventBuffer.cs

@@ -52,6 +52,13 @@ public EdgeEvent GetEvent(ulong index)
         return CallLibgpiod(() =>
         {
             using EdgeEventNotFreeable edgeEventHandle = LibgpiodV2.gpiod_edge_event_buffer_get_event(Handle, index);
+            // gpiod_edge_event_buffer_get_event returns NULL when the requested index is not populated in the buffer. Passing a null pointer
+            // to gpiod_edge_event_copy triggers a native assertion (assert(event)) which aborts the process. Guard against it here.
+            if (edgeEventHandle.IsInvalid)
+            {
+                throw new GpiodException($"Edge event at index {index} is not populated in the buffer.");
+            }
+
             // Since events are tied to the buffer instance, different threads may not operate on the buffer and any associated events at the same
             // time. Events can be copied using ::gpiod_edge_event_copy in order to create a standalone objects - which each may safely be used from
             // a different thread concurrently.

src/System.Device.Gpio/System/Device/Gpio/Drivers/LibGpiodV2EventObserver.cs

@@ -219,10 +219,15 @@ private void HandleEdgeEventsOfRequestInLoop(LineRequest request)
 
                 int numberOfReadEvents = request.ReadEdgeEvents(edgeEventBuffer);
 
-                for (int i = 0; i < numberOfReadEvents; i++)
+                // Bound iteration by the number of events actually stored in the buffer. The value returned by ReadEdgeEvents can exceed the
+                // number of events retrievable from the buffer, in which case GetEvent would receive a null event from libgpiod.
+                int numberOfBufferedEvents = edgeEventBuffer.GetNumEvents();
+                int numberOfEventsToHandle = Math.Min(numberOfReadEvents, numberOfBufferedEvents);
+
+                for (int i = 0; i < numberOfEventsToHandle; i++)
                 {
-                    EdgeEvent edgeEvent = edgeEventBuffer.GetEvent((ulong)i);
-                    HandleEdgeEvent(edgeEvent);
+using EdgeEvent edgeEvent = edgeEventBuffer.GetEvent((ulong)i);
+HandleEdgeEvent(edgeEvent);
                 }
             }
         }