net/sched: move NULL ptr check to qdisc_put() too

In commit 92833e8b titled
"net: sched: rename qdisc_destroy() to qdisc_put()" part of the
functionality of qdisc_destroy() was moved into a (for linux-4.19.y)
new function qdisk_put(), and the previous calls to qdisc_destroy()
were changed to qdisk_put().
This made it similar to f.e. 5.10.y and current master.

There was one part of qdisc_destroy() not moved over to qdisc_put() and
that was the check for a NULL pointer, causing oopses.
(See upstream commit: 6efb971ba8edfbd80b666f29de12882852f095ae)
This patch fixes that.

Fixes: 92833e8b
Reported-by: Thorsten Glaser <tg@mirbsd.de>
Link: https://bugs.debian.org/1013299


Signed-off-by: Diederik de Haas <didi.debian@cknow.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>