Fix critical warnings when using buffer tracer

[alehed]

This fixes a copy-paste mistake that caused a bunch of critical warnings:

GStreamer-CRITICAL **: 11:38:05.911: gst_buffer_get_size: assertion 'GST_IS_BUFFER (buffer)' failed

GStreamer-CRITICAL **: 11:38:05.911: gst_buffer_get_size: assertion 'GST_IS_BUFFER (buffer)' failed

GStreamer-CRITICAL **: 11:38:05.911: gst_buffer_get_size: assertion 'GST_IS_BUFFER (buffer)' failed

GStreamer-CRITICAL **: 11:38:05.911: gst_buffer_get_size: assertion 'GST_IS_BUFFER (buffer)' failed

GStreamer-CRITICAL **: 11:38:05.911: gst_buffer_get_size: assertion 'GST_IS_BUFFER (buffer)' failed

GStreamer-CRITICAL **: 11:38:05.911: gst_buffer_get_size: assertion 'GST_IS_BUFFER (buffer)' failed

This is caused because buffer lists are passed to the function meant to process buffers. This causes at least the buffer tracer to not work for buffer lists.