v4l2transform: don't segfault if flushed without pools
v4l2transform: don't segfault if flushed without pools

The v4l2output and v4l2capture v4l2objects can have pool == NULL if they
have been stopped before.

GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good