Fixing access_log structure reference counting.

The reference to the access_log structure is stored in the current
nxt_router_conf_t and the global nxt_router_t.  When the reference is copied,
the reference counter should be adjusted accordingly.

This closes #593 issue on GitHub.

docs/changes.xml

@@ -31,6 +31,13 @@ NGINX Unit updated to 1.27.0.
          date="" time=""
          packager="Andrei Belov <defan@nginx.com>">
 
+<change type="bugfix">
+<para>
+the router process crash on reconfiguration if "access_log" was configured
+without listeners.
+</para>
+</change>
+
 </changes>