fix default root directory logging path

Currently, log files will be written into root directory if we only
touch /etc/thermal.d/logging.

Before fix:
$ mkdir -p /etc/thermal.d && touch /etc/thermal.d/logging && systemctl restart phosphor-pid-control.service
$ systemctl status -n 10000 phosphor-pid-control.service | grep enabled
swampd[10391]: Logging enabled:
$ ls /zone*
zone_0.log  zone_1.log  zone_2.log

After fix:
Logging enabled: /tmp
$ ls /tmp/zone*
/tmp/zone_0.log  /tmp/zone_1.log  /tmp/zone_2.log

Change-Id: I9f3e93b2d9c469b122f106fab54f3686524ec209
Signed-off-by: Jinliang Wang <jinliangw@google.com>
diff --git a/main.cpp b/main.cpp
index 5c26f01..bc70506 100644
--- a/main.cpp
+++ b/main.cpp
@@ -287,6 +287,10 @@
         {
             loggingPath = altPath;
         }
+        else
+        {
+            loggingPath = defLoggingPath;
+        }
 
         loggingEnabled = true;
     }