avcodec/encode: Fix default setting of bits_per_raw_sample

bits_per_raw_sample doesn't always match size of AV_SAMPLE_FMT_,
e.g., S24.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavcodec/encode.c

@@ -736,6 +736,8 @@ static int encode_preinit_audio(AVCodecContext *avctx)
         }
     }
 
+    if (!avctx->bits_per_raw_sample)
+        avctx->bits_per_raw_sample = av_get_exact_bits_per_sample(avctx->codec_id);
     if (!avctx->bits_per_raw_sample)
         avctx->bits_per_raw_sample = 8 * av_get_bytes_per_sample(avctx->sample_fmt);