hwcontext_d3d11va: properly reset values after release/close

Makes the uninit function re-entrable, which can be a common case
when an API user first tries to initialize its context, fails, and
then finally unrefs the AVHWDevice.

Fixes a crash reported by sm2345 on IRC.

libavutil/hwcontext_d3d11va.c

@@ -458,20 +458,31 @@ static void d3d11va_device_uninit(AVHWDeviceContext *hwdev)
 {
     AVD3D11VADeviceContext *device_hwctx = hwdev->hwctx;
 
-    if (device_hwctx->device)
+    if (device_hwctx->device) {
         ID3D11Device_Release(device_hwctx->device);
+        device_hwctx->device = NULL;
+    }
 
-    if (device_hwctx->device_context)
+    if (device_hwctx->device_context) {
         ID3D11DeviceContext_Release(device_hwctx->device_context);
+        device_hwctx->device_context = NULL;
+    }
 
-    if (device_hwctx->video_device)
+    if (device_hwctx->video_device) {
         ID3D11VideoDevice_Release(device_hwctx->video_device);
+        device_hwctx->video_device = NULL;
+    }
 
-    if (device_hwctx->video_context)
+    if (device_hwctx->video_context) {
         ID3D11VideoContext_Release(device_hwctx->video_context);
+        device_hwctx->video_context = NULL;
+    }
 
-    if (device_hwctx->lock == d3d11va_default_lock)
+    if (device_hwctx->lock == d3d11va_default_lock) {
         CloseHandle(device_hwctx->lock_ctx);
+        device_hwctx->lock_ctx = INVALID_HANDLE_VALUE;
+        device_hwctx->lock = NULL;
+    }
 }
 
 static int d3d11va_device_create(AVHWDeviceContext *ctx, const char *device,