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.

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 36f00f6..7a29608 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -4489,7 +4489,9 @@
     clcf->alias = alias ? clcf->name.len : 0;
     clcf->root = value[1];
 
-    if (!alias && clcf->root.data[clcf->root.len - 1] == '/') {
+    if (!alias && clcf->root.len > 0
+        && clcf->root.data[clcf->root.len - 1] == '/')
+    {
         clcf->root.len--;
     }