[FIXED JENKINS-20407] /var/run/jenkins may not exist

Test for the existence of /var/run/jenkins before trying to chown it in
the Debian post-install script.

debian/debian/jenkins.postinst

@@ -52,11 +52,16 @@ case "$1" in
         chmod u+rwx /var/lib/jenkins /var/log/jenkins
 
         # make sure jenkins can delete everything in /var/cache/jenkins to
-        # re-explode war. older installations may use /var/run/jenkins
+        # re-explode war.
+        chown -R $JENKINS_USER:adm /var/cache/jenkins
+        chmod -R 750               /var/cache/jenkins
+
+        # older installations may use /var/run/jenkins
         # so make sure that they can delete too.
-        mkdir -p /var/run/jenkins
-        chown -R $JENKINS_USER:adm /var/cache/jenkins /var/run/jenkins
-        chmod -R 750               /var/cache/jenkins /var/run/jenkins
+        if test -d /var/run/jenkins ; then
+            chown -R $JENKINS_USER:adm /var/run/jenkins
+            chmod -R 750               /var/run/jenkins
+        fi
     ;;
 
     abort-upgrade|abort-remove|abort-deconfigure)