riff: Pass block_align to estimate frame duration

Fix incorrect wSamplesPerBlock(=0) written for ADPCM_IMA_WAV
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/riffenc.c

@@ -62,7 +62,7 @@ int ff_put_wav_header(AVIOContext *pb, AVCodecContext *enc)
     /* We use the known constant frame size for the codec if known, otherwise
      * fall back on using AVCodecContext.frame_size, which is not as reliable
      * for indicating packet duration. */
-    frame_size = av_get_audio_frame_duration(enc, 0);
+    frame_size = av_get_audio_frame_duration(enc, enc->block_align);
     if (!frame_size)
         frame_size = enc->frame_size;