avcodec/utils: Check bitrate for overflow in get_bit_rate()

Fixes: signed integer overflow: 617890810133996544 * 16 cannot be represented in type 'long'
Fixes: 26565/clusterfuzz-testcase-minimized-ffmpeg_dem_MV_fuzzer-5092054700654592

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

libavcodec/utils.c

@@ -513,7 +513,14 @@ 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 * (int64_t)ctx->channels * bits_per_sample : ctx->bit_rate;
+        if (bits_per_sample) {
+            bit_rate = ctx->sample_rate * (int64_t)ctx->channels;
+            if (bit_rate > INT64_MAX / bits_per_sample) {
+                bit_rate = 0;
+            } else
+                bit_rate *= bits_per_sample;
+        } else
+            bit_rate = ctx->bit_rate;
         break;
     default:
         bit_rate = 0;