v4l2src: Ensure internal pool activation

Before we would hit an assertion "'gst_buffer_pool_is_active (bpool)' failed"
if the internal pool was not used to push buffer downstrea, hence not
given to the baseclass.

https://bugzilla.gnome.org/show_bug.cgi?id=732912
examples/*-rtpaux: Release reference to parent when done

Fixes
tests/icles/videocrop-test.c: Release reference to pipeline

Fixes