lavc/libopusdec: Allow avcodec_open2 to call .close

If there is a decoder initialization failure detected in avcodec_open2
after .init is called, allow graceful decoder .close to prevent leaking
libopus decoder allocations.

BUG=828526
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/libopusdec.c

@@ -160,7 +160,10 @@ static av_cold int libopus_decode_close(AVCodecContext *avc)
 {
     struct libopus_context *opus = avc->priv_data;
 
+    if (opus->dec) {
         opus_multistream_decoder_destroy(opus->dec);
+        opus->dec = NULL;
+    }
     return 0;
 }
 
@@ -252,6 +255,7 @@ AVCodec ff_libopus_decoder = {
     .decode         = libopus_decode,
     .flush          = libopus_flush,
     .capabilities   = AV_CODEC_CAP_DR1,
+    .caps_internal  = FF_CODEC_CAP_INIT_CLEANUP,
     .sample_fmts    = (const enum AVSampleFormat[]){ AV_SAMPLE_FMT_FLT,
                                                      AV_SAMPLE_FMT_S16,
                                                      AV_SAMPLE_FMT_NONE },