vpu: fix gst_buffer that was not set to NULL after unref

This was causing some warnings and segmentation fault when you try to
stop and start the Gstreamer pipeline:

GStreamer-CRITICAL **: gst_mini_object_unref: assertion 'mini_object
>refcount > 0' failed
GStreamer-CRITICAL **: gst_object_unref: assertion '((GObject *)
object)->ref_count > 0' failed

Signed-off-by: Gabriel Huau <gabriel.huau@freescale.com>