Fix crash caused by race condition in timer creation

The race condition is the timer elapses before isc__timer_create()
returns the pointer to the caller.  Assigning the return pointer before
enabling the timer will fix it.

(cherry picked from commit 21966423)
14 jobs for 556-race-condition-in-timer-creation-v9_12 in 8 minutes and 24 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Precheck
passed precheck:debian:sid:amd64 #59041
amd64 docker linux

00:00:19

 
  Build
passed build:centos:centos6:amd64 #59042
amd64 docker linux

00:02:36

passed build:centos:centos7:amd64 #59043
amd64 docker linux

00:02:47

passed build:debian:sid:amd64 #59044
amd64 docker linux

00:02:07

passed build:debian:sid:i386 #59045
i386 docker linux

00:01:43

 
  Test
passed install:debian:sid:amd64 #59054
amd64 docker linux

00:03:22

passed systemtest:centos:centos6:amd64 #59050
amd64 docker linux

00:04:55

passed systemtest:centos:centos7:amd64 #59051
amd64 docker linux

00:04:54

passed systemtest:debian:sid:amd64 #59052
amd64 docker linux

00:04:48

passed systemtest:debian:sid:i386 #59053
i386 docker linux

00:04:47

passed unittest:centos:centos6:amd64 #59046
amd64 docker linux

00:00:56

passed unittest:centos:centos7:amd64 #59047
amd64 docker linux

00:01:03

passed unittest:debian:sid:amd64 #59048
amd64 docker linux

00:00:58

passed unittest:debian:sid:i386 #59049
i386 docker linux

00:00:59