Fixed handling of empty root.
Fixed handling of empty root.

Previously, a configuration with "root" set to an empty string tried test
a character at clcf->root.data[-1].  Additional test added to make sure
this won't happen.

Reported by Markus Linnala, see
http://mailman.nginx.org/pipermail/nginx-devel/2015-August/007210.html.

hy0kl/nginx,hy0kl/nginx,hy0kl/nginx