avcodec/alsdec: Check bitstream input in read_block()

Fixes: Timeout
Fixes: 28110/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5036338973507584

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

@@ -1016,6 +1016,10 @@ static int read_block(ALSDecContext *ctx, ALSBlockData *bd)
     ALSSpecificConfig *sconf = &ctx->sconf;
 
     *bd->shift_lsbs = 0;
+
+    if (get_bits_left(gb) < 1)
+        return AVERROR_INVALIDDATA;
+
     // read block type flag and read the samples accordingly
     if (get_bits1(gb)) {
         ret = read_var_block_data(ctx, bd);