[sysprof] libsysprof-capture: fix advancement in drain callback

[Christian Hergert <chergert src gnome org>]

commit ed7c9bbaeff9d889f9632e70c3c69327cfd03b24
Author: Christian Hergert <chergert redhat com>
Date:   Sat Feb 15 22:02:03 2020 -0700

    libsysprof-capture: fix advancement in drain callback

 src/libsysprof/sysprof-control-source.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/src/libsysprof/sysprof-control-source.c b/src/libsysprof/sysprof-control-source.c
index 04d8ec3..4ccdc48 100644
--- a/src/libsysprof/sysprof-control-source.c
+++ b/src/libsysprof/sysprof-control-source.c
@@ -142,6 +142,8 @@ event_frame_cb (gconstpointer  data,
 
   _sysprof_capture_writer_add_raw (rd->self->writer, fr);
 
+  *length = fr->len;
+
   return G_SOURCE_CONTINUE;
 
 remove_source: