avutil/frame: Return 0 on success in av_frame_ref()

av_frame_copy() is allowed to return values >= 0 on success, whereas
the documentation of av_frame_ref() states that the return value is 0 on
success. Ergo the latter must not just return the former's return value.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/frame.c

@@ -469,7 +469,7 @@ int av_frame_ref(AVFrame *dst, const AVFrame *src)
         if (ret < 0)
             goto fail;
 
-        return ret;
+        return 0;
     }
 
     /* ref the buffers */