flacdec: replace -1 with AV_SAMPLE_FMT_NONE

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/flacdec.c

@@ -573,5 +573,5 @@ AVCodec ff_flac_decoder = {
                                                       AV_SAMPLE_FMT_S16P,
                                                       AV_SAMPLE_FMT_S32,
                                                       AV_SAMPLE_FMT_S32P,
-                                                      -1 },
+                                                      AV_SAMPLE_FMT_NONE },
 };