avutil/hwcontext_qsv: register free function for device_derive

When qsv device is created by device_derive, the ctx->free function is
not registered, causing potential memory leak because of not properly
closing the MFX session.

Signed-off-by: Tong Wu <tong1.wu@intel.com>
Signed-off-by: Wenbin Chen <wenbin.chen@intel.com>

libavutil/hwcontext_qsv.c

@@ -2094,6 +2094,15 @@ static int qsv_device_derive(AVHWDeviceContext *ctx,
                              AVDictionary *opts, int flags)
 {
     mfxIMPL impl;
+    QSVDevicePriv *priv;
+
+    priv = av_mallocz(sizeof(*priv));
+    if (!priv)
+        return AVERROR(ENOMEM);
+
+    ctx->user_opaque = priv;
+    ctx->free = qsv_device_free;
+
     impl = choose_implementation("hw_any", child_device_ctx->type);
     return qsv_device_derive_from_child(ctx, impl,
                                         child_device_ctx, flags);