avcodec/cbs: Fix potential double-free when adding unit fails

ff_cbs_insert_unit_data() has two modes of operation: It can insert a
unit with a newly created reference to an already existing AVBuffer; or
it can take a buffer and create an AVBuffer for it. Said buffer will
then become owned by the unit lateron.

A potential memleak/double-free exists in the second case, because if
creating the AVBuffer fails, the function immediately returns, but when
it fails lateron, the supplied buffer will be freed. The caller has no
way to distinguish between these two outcomes. The only such caller
(cbs_jpeg_split_fragment() in cbs_jpeg.c) opted for a potential
double-free.

This commit changes this by explicitly stating that a non-refcounted
buffer will be freed on error. The aforementioned caller has been
brought in line with this.

Fixes CID 1452623.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/cbs.c

@@ -775,8 +775,11 @@ int ff_cbs_insert_unit_data(CodedBitstreamContext *ctx,
         data_ref = av_buffer_ref(data_buf);
     else
         data_ref = av_buffer_create(data, data_size, NULL, NULL, 0);
-    if (!data_ref)
+    if (!data_ref) {
+        if (!data_buf)
+            av_free(data);
         return AVERROR(ENOMEM);
+    }
 
     err = cbs_insert_unit(ctx, frag, position);
     if (err < 0) {

libavcodec/cbs.h

@@ -376,7 +376,8 @@ int ff_cbs_insert_unit_content(CodedBitstreamContext *ctx,
  * Insert a new unit into a fragment with the given data bitstream.
  *
  * If data_buf is not supplied then data must have been allocated with
- * av_malloc() and will become owned by the unit after this call.
+ * av_malloc() and will on success become owned by the unit after this
+ * call or freed on error.
  */
 int ff_cbs_insert_unit_data(CodedBitstreamContext *ctx,
                             CodedBitstreamFragment *frag,

libavcodec/cbs_jpeg.c

@@ -225,11 +225,8 @@ static int cbs_jpeg_split_fragment(CodedBitstreamContext *ctx,
 
         err = ff_cbs_insert_unit_data(ctx, frag, unit, marker,
                                       data, data_size, data_ref);
-        if (err < 0) {
-            if (!data_ref)
-                av_freep(&data);
+        if (err < 0)
             return err;
-        }
 
         if (next_marker == -1)
             break;