sub/lavc_conv: free avctx correctly on error

Fixes memory leak on error.

sub/lavc_conv.c

@@ -107,7 +107,7 @@ struct lavc_conv *lavc_conv_create(struct sd *sd)
  error:
     MP_FATAL(priv, "Could not open libavcodec subtitle converter\n");
     av_dict_free(&opts);
-    av_free(avctx);
+    avcodec_free_context(&avctx);
     mp_free_av_packet(&priv->avpkt);
     mp_free_av_packet(&priv->avpkt_vtt);
     talloc_free(priv);