avcodec/alsdec: No channels cannot be accessed

Fixes: out of array access
Fixes: 48145/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5054524173189120

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

libavcodec/alsdec.c

@@ -361,6 +361,9 @@ static av_cold int read_specific_config(ALSDecContext *ctx)
         return AVERROR_PATCHWELCOME;
     }
 
+    if (avctx->ch_layout.nb_channels == 0)
+        return AVERROR_INVALIDDATA;
+
     ctx->cur_frame_length = sconf->frame_length;
 
     // read channel config