rgw: set default value for env->get() call

Fixes: ceph#13239

This fixes a regression introduced at commit abe4ec2.
The host var is a string, env->get() returns a char pointer, shouldn't
pass in NULL.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>

src/rgw/rgw_common.cc

@@ -99,7 +99,7 @@ is_err() const
 
 
 req_info::req_info(CephContext *cct, class RGWEnv *e) : env(e) {
-  method = env->get("REQUEST_METHOD");
+  method = env->get("REQUEST_METHOD", "");
   script_uri = env->get("SCRIPT_URI", cct->_conf->rgw_script_uri.c_str());
   request_uri = env->get("REQUEST_URI", cct->_conf->rgw_request_uri.c_str());
   int pos = request_uri.find('?');
@@ -109,7 +109,7 @@ req_info::req_info(CephContext *cct, class RGWEnv *e) : env(e) {
   } else {
     request_params = env->get("QUERY_STRING", "");
   }
-  host = env->get("HTTP_HOST");
+  host = env->get("HTTP_HOST", "");
 
   // strip off any trailing :port from host (added by CrossFTP and maybe others)
   size_t colon_offset = host.find_last_of(':');