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_11 in 7 minutes and 35 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Precheck
passed precheck:debian:sid:amd64 #59055
amd64 docker linux

00:00:18

 
  Build
passed build:centos:centos6:amd64 #59056
linux amd64 docker

00:02:10

passed build:centos:centos7:amd64 #59057
amd64 docker linux

00:02:08

passed build:debian:sid:amd64 #59058
amd64 docker linux

00:01:44

passed build:debian:sid:i386 #59059
i386 docker linux

00:01:27

 
  Test
passed install:debian:sid:amd64 #59068
amd64 docker linux

00:02:13

passed systemtest:centos:centos6:amd64 #59064
amd64 docker linux

00:04:37

passed systemtest:centos:centos7:amd64 #59065
amd64 docker linux

00:04:36

passed systemtest:debian:sid:amd64 #59066
amd64 docker linux

00:04:25

passed systemtest:debian:sid:i386 #59067
i386 docker linux

00:05:04

passed unittest:centos:centos6:amd64 #59060
amd64 docker linux

00:01:04

passed unittest:centos:centos7:amd64 #59061
amd64 docker linux

00:00:54

passed unittest:debian:sid:amd64 #59062
amd64 docker linux

00:01:02

passed unittest:debian:sid:i386 #59063
i386 docker linux

00:00:55