adpcmenc: Use correct frame_size for Yamaha ADPCM.

Output packet size should match avctx->block_align. The target output packet
size is 1024 bytes.
Before:
mono   - 1024 samples -> 512 bytes
stereo - 2048 samples -> 2048 bytes
After:
mono   - 2048 samples -> 1024 bytes
stereo - 1024 samples -> 1024 bytes

libavcodec/adpcmenc.c

@@ -122,7 +122,7 @@ static av_cold int adpcm_encode_init(AVCodecContext *avctx)
         }
         break;
     case CODEC_ID_ADPCM_YAMAHA:
-        avctx->frame_size  = BLKSIZE * avctx->channels;
+        avctx->frame_size  = BLKSIZE * 2 / avctx->channels;
         avctx->block_align = BLKSIZE;
         break;
     case CODEC_ID_ADPCM_SWF:

tests/ref/acodec/adpcm_yam

-006f8dc92eb4f7bab82eded314ca1124 *./tests/data/acodec/adpcm_yam.wav
-266298 ./tests/data/acodec/adpcm_yam.wav
-c36a9d5a1e0ad57fbe9665a31373b7c1 *./tests/data/adpcm_yam.acodec.out.wav
-stddev: 1247.60 PSNR: 34.41 MAXDIFF:39895 bytes:  1064960/  1058400
+e9c14f701d25947317db9367b9dc772d *./tests/data/acodec/adpcm_yam.wav
+265274 ./tests/data/acodec/adpcm_yam.wav
+1488b5974fa040a65f0d407fc0224c6a *./tests/data/adpcm_yam.acodec.out.wav
+stddev: 1247.60 PSNR: 34.41 MAXDIFF:39895 bytes:  1060864/  1058400