lpcm: prepare dec->fmt_out

In order to set i_bytes_per_frame and i_frame_length.

Fixes #20753

modules/codec/lpcm.c

@@ -455,6 +455,7 @@ static block_t *Packetize( decoder_t *p_dec, block_t **pp_block )
             p_dec->fmt_out.i_codec = VLC_CODEC_S32N;
             p_dec->fmt_out.audio.i_bitspersample = 32;
         }
+        aout_FormatPrepare(&p_dec->fmt_out.audio);
 
         /* */
         block_t *p_aout_buffer;