avcodec/parser: Don't return pointer to stack buffer

When flushing, the parser receives a dummy buffer with padding
that lives on the stack of av_parser_parse2(). Certain parsers
(e.g. Dolby E) only analyze the input, but don't repack it. When
flushing, such parsers return a pointer to the stack buffer and
a size of 0. And this is also what av_parser_parse2() returns.

Fix this by always resetting poutbuf in case poutbuf_size is zero.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 9faf3f8bb0)

libavcodec/parser.c

@@ -179,6 +179,9 @@ int av_parser_parse2(AVCodecParserContext *s, AVCodecContext *avctx,
         /* offset of the next frame */
         s->next_frame_offset = s->cur_offset + index;
         s->fetch_timestamp   = 1;
+    } else {
+        /* Don't return a pointer to dummy_buf. */
+        *poutbuf = NULL;
     }
     if (index < 0)
         index = 0;