closedir() only if opendir() succeeds

Fix compiler warning:
conf.c:186:3: warning: Null pointer passed as an argument to a 'nonnull' parameter
  closedir(d);
  ^        ~

libnfc/conf.c

@@ -182,8 +182,8 @@ conf_devices_load(const char *dirname, nfc_context *context)
         }
       }
     }
+	closedir(d);
   }
-  closedir(d);
 }
 
 void