lavr: return an error if a avresample_open() is called on an open context

libavresample/utils.c

@@ -36,6 +36,11 @@ int avresample_open(AVAudioResampleContext *avr)
 {
     int ret;
 
+    if (avresample_is_open(avr)) {
+        av_log(avr, AV_LOG_ERROR, "The resampling context is already open.\n");
+        return AVERROR(EINVAL);
+    }
+
     /* set channel mixing parameters */
     avr->in_channels = av_get_channel_layout_nb_channels(avr->in_channel_layout);
     if (avr->in_channels <= 0 || avr->in_channels > AVRESAMPLE_MAX_CHANNELS) {