Small fix on the top of #950 to sanity check the variable before use it

Author: Felipe Zimmerle

nginx/modsecurity/ngx_http_modsecurity.c

@@ -364,7 +364,12 @@ ngx_http_modsecurity_load_request(ngx_http_request_t *r)
     /*      &r->headers_in.user);                           */
     req->parsed_uri.fragment = (char *)ngx_pstrdup0(r->pool, &r->exten);
 
-    req->hostname = (char *)ngx_pstrdup0(r->pool, &r->headers_in.server);
+    if (&r->headers_in != NULL && &r->headers_in.server != NULL) {
+        req->hostname = (char *)ngx_pstrdup0(r->pool, &r->headers_in.server);
+    } else {
+        req->hostname = (char *)ngx_pstrdup0(r->pool,
+            (ngx_str_t *)&ngx_cycle->hostname);
+    }
 
     if (r->header_only) {
         req->header_only = r->header_only;