vulkan_decode: fix small memory leak

This requires using the new AVHWFramesContext.opaque field, as
otherwise, the profile attached to the decoder will be freed
before the frames context, rendering the frames context useless.

libavcodec/vulkan_decode.c

@@ -584,6 +584,8 @@ static void free_common(void *opaque, uint8_t *data)
                                           s->hwctx->alloc);
 
     ff_vk_uninit(s);
+
+    av_free(ctx);
 }
 
 static int vulkan_decode_bootstrap(AVCodecContext *avctx, AVBufferRef *frames_ref)