sipr: fall back to setting mode based on bit_rate.

Not all applications (e.g. MPlayer) set block_align, and
when using a different demuxer it might not even be
easily available.
So fall back to selecting mode based on bit rate as before
if block_align has not useful value.
It can't be worse than failing to decode completely.

(cherry picked from commit 1d0d63052b)

CC: libav-stable@libav.org
Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/sipr.c

@@ -485,8 +485,13 @@ static av_cold int sipr_decoder_init(AVCodecContext * avctx)
     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);
+        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;
+        av_log(avctx, AV_LOG_WARNING,
+               "Invalid block_align: %d. Mode %s guessed based on bitrate: %d\n",
+               avctx->block_align, modes[ctx->mode].mode_name, avctx->bit_rate);
     }
 
     av_log(avctx, AV_LOG_DEBUG, "Mode: %s\n", modes[ctx->mode].mode_name);