v4l2allocator: Don't do REQBUFS(0) on inactive allocator
v4l2allocator: Don't do REQBUFS(0) on inactive allocator

If the allocator is no longer active, it means the memory has already
been freed, calling REQBUF(0) would make no sense.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/649>

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