sipr: set mode based on block_align instead of bit_rate

the user is not required to set bit_rate

libavcodec/sipr.c

@@ -480,10 +480,15 @@ static av_cold int sipr_decoder_init(AVCodecContext * avctx)
     SiprContext *ctx = avctx->priv_data;
     int i;
 
-    if      (avctx->bit_rate > 12200) ctx->mode = MODE_16k;
-    else if (avctx->bit_rate > 7500 ) ctx->mode = MODE_8k5;
-    else if (avctx->bit_rate > 5750 ) ctx->mode = MODE_6k5;
-    else                              ctx->mode = MODE_5k0;
+    switch (avctx->block_align) {
+    case 20: ctx->mode = MODE_16k; break;
+    case 19: ctx->mode = MODE_8k5; break;
+    case 29: ctx->mode = MODE_6k5; break;
+    case 37: ctx->mode = MODE_5k0; break;
+    default:
+        av_log(avctx, AV_LOG_ERROR, "Invalid block_align: %d\n", avctx->block_align);
+        return AVERROR(EINVAL);
+    }
 
     av_log(avctx, AV_LOG_DEBUG, "Mode: %s\n", modes[ctx->mode].mode_name);