alsdec/read_specific_config: check for init_get_bits failure

This also fixes a potential integer overflow
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Thilo Borgmann <thilo.borgmann@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/alsdec.c

@@ -289,8 +289,10 @@ static av_cold int read_specific_config(ALSDecContext *ctx)
     ALSSpecificConfig *sconf = &ctx->sconf;
     AVCodecContext *avctx    = ctx->avctx;
     uint32_t als_id, header_size, trailer_size;
+    int ret;
 
-    init_get_bits(&gb, avctx->extradata, avctx->extradata_size * 8);
+    if ((ret = init_get_bits8(&gb, avctx->extradata, avctx->extradata_size)) < 0)
+        return ret;
 
     config_offset = avpriv_mpeg4audio_get_config(&m4ac, avctx->extradata,
                                                  avctx->extradata_size * 8, 1);