[configfile] fix reading uninitialized variable (found by Willian B.)

[configfile] fix reading uninitialized variable (found by Willian B.)

- stream_open()-ing an empty file shouldn't return an error (and didn't on my
  system)
- don't try to handle empty file as non-error in config_parse_file;
  this fixes the read of an potentially unitialized variable
- stream_open()-ing an empty file doesn't try to map the file anymore
  and should not result in any errors; return an empty stream instead.
- stream_open(): make sure the returned stream is always initialized
  correctly, and can always be used with stream_close(), whether opening
  was successful or not
- stream_close(): also reset the size member

From: Stefan Bühler <stbuehler@web.de>

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@3003 152afb58-edef-0310-8abb-c4023f1b3aa9