avcodec/alsdec: Check opt_order / sb_length in ra_block handling

Fixes: out of array access
Fixes: 15277/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5184853437317120
Fixes: 15280/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5741062137577472

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

libavcodec/alsdec.c

@@ -789,14 +789,20 @@ static int read_var_block_data(ALSDecContext *ctx, ALSBlockData *bd)
 
     // read first value and residuals in case of a random access block
     if (bd->ra_block) {
+        start = FFMIN(opt_order, 3);
+        av_assert0(sb_length <= sconf->frame_length);
+        if (sb_length <= start) {
+            // opt_order or sb_length may be corrupted, either way this is unsupported and not well defined in the specification
+            av_log(avctx, AV_LOG_ERROR, "Sub block length smaller or equal start\n");
+            return AVERROR_PATCHWELCOME;
+        }
+
         if (opt_order)
             bd->raw_samples[0] = decode_rice(gb, avctx->bits_per_raw_sample - 4);
         if (opt_order > 1)
             bd->raw_samples[1] = decode_rice(gb, FFMIN(s[0] + 3, ctx->s_max));
         if (opt_order > 2)
             bd->raw_samples[2] = decode_rice(gb, FFMIN(s[0] + 1, ctx->s_max));
-
-        start = FFMIN(opt_order, 3);
     }
 
     // read all residuals