s3/winbindd: use == -1 instead of < 0 for error checking uid_t

s3/winbindd: use == -1 instead of < 0 for error checking uid_t

The sign of the uid_t type is left unspecified by POSIX. It's defined as
an unsigned 32b int on Linux, therefore the < 0 check is always
false.

For unsigned version of uid_t, "uid == -1" will implicitely cast -1 to
unsigned making it a valid test for both signed and unsigned version of
uid_t.

This commit makes the cast to (uid_t) explicit anyway.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>
Autobuild-User(master): Michael Adam <obnox@samba.org>
Autobuild-Date(master): Fri Jul  1 05:22:36 CEST 2016 on sn-devel-144