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 (!out) {
-        av_frame_free(&in);
-        return AVERROR(ENOMEM);
+    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);
     }
-    av_frame_copy_props(out, in);
 
     if (s->limitergain > 0) {
         for (i = 0; i < inlink->channels; i++) {
@@ -134,7 +139,8 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
         }
     }
 
-    av_frame_free(&in);
+    if (out != in)
+        av_frame_free(&in);
     return ff_filter_frame(outlink, out);
 }
 static const AVFilterPad dcshift_inputs[] = {