lavc: make avcodec_close() work properly on unopened codecs.

I.e. free the priv_data and other stuff allocated in
avcodec_alloc_context3() and not segfault.

libavcodec/avcodec.h

@@ -3417,7 +3417,8 @@ int avcodec_get_context_defaults3(AVCodecContext *s, AVCodec *codec);
 
 /**
  * Allocate an AVCodecContext and set its fields to default values.  The
- * resulting struct can be deallocated by simply calling av_free().
+ * resulting struct can be deallocated by calling avcodec_close() on it followed
+ * by av_free().
  *
  * @param codec if non-NULL, allocate private data and initialize defaults
  *              for the given codec. It is illegal to then call avcodec_open2()
@@ -3809,6 +3810,15 @@ int avcodec_encode_video(AVCodecContext *avctx, uint8_t *buf, int buf_size,
 int avcodec_encode_subtitle(AVCodecContext *avctx, uint8_t *buf, int buf_size,
                             const AVSubtitle *sub);
 
+/**
+ * Close a given AVCodecContext and free all the data associated with it
+ * (but not the AVCodecContext itself).
+ *
+ * Calling this function on an AVCodecContext that hasn't been opened will free
+ * the codec-specific data allocated in avcodec_alloc_context3() /
+ * avcodec_get_context_defaults3() with a non-NULL codec. Subsequent calls will
+ * do nothing.
+ */
 int avcodec_close(AVCodecContext *avctx);
 
 /**

libavcodec/utils.c

@@ -1267,14 +1267,17 @@ av_cold int avcodec_close(AVCodecContext *avctx)
         return -1;
     }
 
-    if (HAVE_THREADS && avctx->thread_opaque)
-        ff_thread_free(avctx);
-    if (avctx->codec && avctx->codec->close)
-        avctx->codec->close(avctx);
-    avcodec_default_free_buffers(avctx);
-    avctx->coded_frame = NULL;
-    av_freep(&avctx->internal);
-    if (avctx->codec && avctx->codec->priv_class)
+    if (avcodec_is_open(avctx)) {
+        if (HAVE_THREADS && avctx->thread_opaque)
+            ff_thread_free(avctx);
+        if (avctx->codec && avctx->codec->close)
+            avctx->codec->close(avctx);
+        avcodec_default_free_buffers(avctx);
+        avctx->coded_frame = NULL;
+        av_freep(&avctx->internal);
+    }
+
+    if (avctx->priv_data && avctx->codec && avctx->codec->priv_class)
         av_opt_free(avctx->priv_data);
     av_opt_free(avctx);
     av_freep(&avctx->priv_data);