avcodec/huffyuv: remove gray8a (the format is listed but not supported by the implementation)

Fixes: null pointer dereference
Fixes: 15464/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HYMT_fuzzer-5681391150301184

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/huffyuvdec.c

@@ -418,9 +418,6 @@ static av_cold int decode_init(AVCodecContext *avctx)
         case 0x0F0:
             avctx->pix_fmt = AV_PIX_FMT_GRAY16;
             break;
-        case 0x170:
-            avctx->pix_fmt = AV_PIX_FMT_GRAY8A;
-            break;
         case 0x470:
             avctx->pix_fmt = AV_PIX_FMT_GBRP;
             break;

libavcodec/huffyuvenc.c

@@ -268,7 +268,6 @@ FF_ENABLE_DEPRECATION_WARNINGS
     case AV_PIX_FMT_YUVA420P:
     case AV_PIX_FMT_YUVA422P:
     case AV_PIX_FMT_GBRAP:
-    case AV_PIX_FMT_GRAY8A:
     case AV_PIX_FMT_YUV420P9:
     case AV_PIX_FMT_YUV420P10:
     case AV_PIX_FMT_YUV420P12:
@@ -1122,7 +1121,6 @@ AVCodec ff_ffvhuff_encoder = {
         AV_PIX_FMT_GRAY8, AV_PIX_FMT_GRAY16,
         AV_PIX_FMT_YUVA420P, AV_PIX_FMT_YUVA422P, AV_PIX_FMT_YUVA444P,
         AV_PIX_FMT_GBRAP,
-        AV_PIX_FMT_GRAY8A,
         AV_PIX_FMT_YUV420P9, AV_PIX_FMT_YUV420P10, AV_PIX_FMT_YUV420P12, AV_PIX_FMT_YUV420P14, AV_PIX_FMT_YUV420P16,
         AV_PIX_FMT_YUV422P9, AV_PIX_FMT_YUV422P10, AV_PIX_FMT_YUV422P12, AV_PIX_FMT_YUV422P14, AV_PIX_FMT_YUV422P16,
         AV_PIX_FMT_YUV444P9, AV_PIX_FMT_YUV444P10, AV_PIX_FMT_YUV444P12, AV_PIX_FMT_YUV444P14, AV_PIX_FMT_YUV444P16,