avcodec/flacenc: Fix memleak upon init error

An AVMD5 struct would leak if an error happened after its allocation.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 56bd071e54)

libavcodec/flacenc.c

@@ -1513,4 +1513,5 @@ AVCodec ff_flac_encoder = {
                                                      AV_SAMPLE_FMT_S32,
                                                      AV_SAMPLE_FMT_NONE },
     .priv_class     = &flac_encoder_class,
+    .caps_internal  = FF_CODEC_CAP_INIT_CLEANUP,
 };