avfilter/af_dcshift: add direct path

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/af_dcshift.c

@@ -90,11 +90,16 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     int i, j;
     double dcshift = s->dcshift;
 
+    if (av_frame_is_writable(in)) {
+        out = in;
+    } else {
+        out = ff_get_audio_buffer(inlink, in->nb_samples);
         if (!out) {
             av_frame_free(&in);
             return AVERROR(ENOMEM);
         }
         av_frame_copy_props(out, in);
+    }
 
     if (s->limitergain > 0) {
         for (i = 0; i < inlink->channels; i++) {
@@ -134,6 +139,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
         }
     }
 
+    if (out != in)
         av_frame_free(&in);
     return ff_filter_frame(outlink, out);
 }