[fix] [exporterhelper] Fix nil ptr dereference

When persistent queue fails to start, for example due to bad
permissions, then persistent storage does not get allocated.
Thus, ensure stop storage method is only called if actually
initialized.

exporter/exporterhelper/internal/persistent_queue.go

@@ -16,7 +16,9 @@ import (
 var (
 	// Monkey patching for unit test
 	stopStorage = func(queue *persistentQueue) {
-		queue.storage.stop()
+		if queue.storage != nil {
+			queue.storage.stop()
+		}
 	}
 	errNoStorageClient    = errors.New("no storage client extension found")
 	errWrongExtensionType = errors.New("requested extension is not a storage extension")

exporter/exporterhelper/internal/persistent_queue_test.go

@@ -292,3 +292,12 @@ func TestInvalidStorageExtensionType(t *testing.T) {
 	assert.ErrorIs(t, err, errWrongExtensionType)
 	assert.Nil(t, client)
 }
+
+func TestPersistentQueue_StopAfterBadStart(t *testing.T) {
+	pq := NewPersistentQueue(1, 1, component.ID{}, newFakeTracesRequestMarshalerFunc(),
+		newFakeTracesRequestUnmarshalerFunc())
+	// verify that stopping a un-start/started w/error queue does not panic
+	assert.NotPanics(t, func() {
+		pq.Stop()
+	})
+}