vdpau: have av_vdpau_bind_context() fail on unsupported flag

Currently, no flags are supported.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/vdpau.c

@@ -313,6 +313,9 @@ int av_vdpau_bind_context(AVCodecContext *avctx, VdpDevice device,
 {
     VDPAUHWContext *hwctx;
 
+    if (flags != 0)
+        return AVERROR(EINVAL);
+
     if (av_reallocp(&avctx->hwaccel_context, sizeof(*hwctx)))
         return AVERROR(ENOMEM);