avtpsrc: Use correct size for provided buffers

Without this patch the following pipeline would send packets containing
garbage in the data section.
$ gst-launch-1.0 avtpsrc ! avtpsink

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1077>

subprojects/gst-plugins-bad/ext/avtp/gstavtpsrc.c

@@ -303,6 +303,7 @@ retry:
     return GST_FLOW_ERROR;
   }
 
+  gst_buffer_set_size (buffer, received);
   gst_buffer_unmap (buffer, &map);
 
   return GST_FLOW_OK;