avcodec/utils: Fix overflow in get_bit_rates computations

Fixes: 129ca3e28d73af7b1e24a9d4118e7a2d/signal_sigabrt_7ffff6ae7cc9_836_762b310fc3ef6087bd7771e5d8e90b9b.asf

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/utils.c

@@ -1141,7 +1141,7 @@ static int64_t get_bit_rate(AVCodecContext *ctx)
         break;
     case AVMEDIA_TYPE_AUDIO:
         bits_per_sample = av_get_bits_per_sample(ctx->codec_id);
-        bit_rate = bits_per_sample ? ctx->sample_rate * ctx->channels * bits_per_sample : ctx->bit_rate;
+        bit_rate = bits_per_sample ? ctx->sample_rate * (int64_t)ctx->channels * bits_per_sample : ctx->bit_rate;
         break;
     default:
         bit_rate = 0;