Fix closedir usage in virNumaGetPages

virNumaGetPages calls closedir(dir) in cleanup and dir could
be NULL if we jump there from the failed opendir() call.

While it's not harmful on Linux, FreeBSD libc crashes [1], so
make sure that dir is not NULL before calling closedir.

1: http://lists.freebsd.org/pipermail/freebsd-standards/2014-January/002704.html

src/util/virnuma.c

@@ -843,7 +843,8 @@ virNumaGetPages(int node,
     VIR_FREE(tmp_free);
     VIR_FREE(tmp_avail);
     VIR_FREE(tmp_size);
-    closedir(dir);
+    if (dir)
+        closedir(dir);
     VIR_FREE(path);
     return ret;
 }