lavu/vulkan: check for initialization when freeing buffers

What happens on startup is that ffmpeg.c initializes the filter,
then frees it without feeding a single frame through. With no
input frame, the filter lacks a hardware device. The rest of the
uninit code checks if Vulkan objects exist, which they must if there's
a hardware device, but vk->DeviceWaitIdle does not require an object.
So, add a check for it.

libavutil/vulkan.c

@@ -350,7 +350,7 @@ void ff_vk_free_buf(FFVulkanContext *s, FFVkBuffer *buf)
 {
     FFVulkanFunctions *vk = &s->vkfn;
 
-    if (!buf)
+    if (!buf || !s->hwctx)
         return;
 
     vk->DeviceWaitIdle(s->hwctx->act_dev);