s3: util: Do not take over stderr when there is no log file

In case we don't have either a /var/log/samba directory, or pass a
non-existent log directory through '-l' option, all commands that are
daemonized with '-D' option hang when executed within a subshell.

An example on how to trigger that:

  # rm -r /var/log/samba
  # s=$(nmbd -D -s /etc/samba/smb.conf -l /foo123)
  (never returns)

So, when the above command is executed within a subshell the following
happens:

  (a) Parent shell creates a pipe, sets write side of it to fd 1
    (stdout), call read() on read-side fd, forks off a new child process
    and then executes nmbd in it.
  (b) nmbd sets up initial logging to go through fd 1 (stdout) by
    calling setup_logging(..., DEBUG_DEFAULT_STDOUT). 'state.fd' is now
    set to 1.
  (c) reopen_logs() is called by the first time which then calls
    reopen_logs_internal()
  (d) in reopen_logs_internal(), it attempts to create log.nmbd file in
    /foo123 directory and fails because directory doesn't exist.
  (e) Regardless whether the log file was created or not, it calls
    dup2(state.fd, 2) which dups fd 1 into fd 2.
  (f) At some point, fd 0 and 1 are closed and set to /dev/null

The problem with that is because parent shell in (a) is still blocked in
read() call and the new write side of the pipe is now fd 2 -- after
dup2() in (e) -- and remains unclosed.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13578

Signed-off-by: Paulo Alcantara <palcantara@suse.de>
Reviewed-by: Jim McDonough <jmcd@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sat Aug 18 01:32:25 CEST 2018 on sn-devel-144

lib/util/debug.c

@@ -1077,8 +1077,11 @@ bool reopen_logs_internal(void)
 	force_check_log_size();
 	(void)umask(oldumask);
 
-	/* Take over stderr to catch output into logs */
-	if (state.fd > 0) {
+	/*
+	 * If log file was opened or created successfully, take over stderr to
+	 * catch output into logs.
+	 */
+	if (new_fd != -1) {
 		if (dup2(state.fd, 2) == -1) {
 			/* Close stderr too, if dup2 can't point it -
 			   at the logfile.  There really isn't much