avformat/oggparsevorbis: use the base64 decode size macro

Allocate the memory needed for the decoded data rather than the
encoded data.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/oggparsevorbis.c

@@ -152,8 +152,8 @@ int ff_vorbis_comment(AVFormatContext *as, AVDictionary **m,
              * recommended way of embedding cover art within VorbisComments."
              */
             if (!strcmp(tt, "METADATA_BLOCK_PICTURE") && parse_picture) {
-                int ret;
-                char *pict = av_malloc(vl);
+                int ret, len = AV_BASE64_DECODE_SIZE(vl);
+                char *pict = av_malloc(len);
 
                 if (!pict) {
                     av_log(as, AV_LOG_WARNING, "out-of-memory error. Skipping cover art block.\n");
@@ -161,7 +161,7 @@ int ff_vorbis_comment(AVFormatContext *as, AVDictionary **m,
                     av_freep(&ct);
                     continue;
                 }
-                if ((ret = av_base64_decode(pict, ct, vl)) > 0)
+                if ((ret = av_base64_decode(pict, ct, len)) > 0)
                     ret = ff_flac_parse_picture(as, pict, ret);
                 av_freep(&tt);
                 av_freep(&ct);