avcodec/flac_parser: fix handling EOF if no headers are found

Fixes assertion failure
Fixes Ticket4269
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/flac_parser.c

@@ -704,7 +704,7 @@ static int flac_parse(AVCodecParserContext *s, AVCodecContext *avctx,
 handle_error:
     *poutbuf      = NULL;
     *poutbuf_size = 0;
-    return read_end - buf;
+    return buf_size ? read_end - buf : 0;
 }
 
 static av_cold int flac_parse_init(AVCodecParserContext *c)