decoder: fix data race in input_DecoderIsEmpty()

p_dec->fmt_out is owned by the decoder plugin, and can only safely be
accessed by the core from within decoder callbacks, notably the format
update callbacks.

Outside that context, p_owner->fmt has to be used. It contains a copy
of p_dec->fmt_out at the last format update.

src/input/decoder.c

@@ -382,9 +382,9 @@ bool input_DecoderIsEmpty( decoder_t * p_dec )
     {
         vlc_mutex_lock( &p_owner->lock );
         /* TODO subtitles support */
-        if( p_dec->fmt_out.i_cat == VIDEO_ES && p_owner->p_vout )
+        if( p_owner->fmt.i_cat == VIDEO_ES && p_owner->p_vout )
             b_empty = vout_IsEmpty( p_owner->p_vout );
-        else if( p_dec->fmt_out.i_cat == AUDIO_ES && p_owner->p_aout )
+        else if( p_owner->fmt.i_cat == AUDIO_ES && p_owner->p_aout )
             b_empty = aout_DecIsEmpty( p_owner->p_aout );
         vlc_mutex_unlock( &p_owner->lock );
     }