lib: zlog should clean up its memory

* log.c: (closezlog) Also free the dynamically allocated filename when
    a log is closed.
diff --git a/lib/log.c b/lib/log.c
index 0c2f655..df6e13d 100644
--- a/lib/log.c
+++ b/lib/log.c
@@ -653,6 +653,9 @@
   if (zl->fp != NULL)
     fclose (zl->fp);
 
+  if (zl->filename != NULL)
+    free (zl->filename);
+
   XFREE (MTYPE_ZLOG, zl);
 }