sch_sfq: fix null pointer dereference on init failure

Currently only a memory allocation failure can lead to this, so let's
initialize the timer first.

Fixes: 6529eaba ("net: sched: introduce tcf block infractructure")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>