vulkan_decode: check if yuv_sampler exists before freeing it

This prevents multiple NULL accesses - if yuv_sampler exists, then
everything required for it to be destroyed also exists.

libavcodec/vulkan_decode.c

@@ -581,6 +581,7 @@ static void free_common(void *opaque, uint8_t *data)
 
     ff_vk_video_common_uninit(s, &ctx->common);
 
+    if (ctx->yuv_sampler)
         vk->DestroySamplerYcbcrConversion(s->hwctx->act_dev, ctx->yuv_sampler,
                                           s->hwctx->alloc);