nsswitch pam_winbind: Fix Asan use after free

Fix use after free condition detected by Address Sanitizer triggered by
wbcLogonUserInfoDestructor, wbcFreeMemory has code to detect and prevent a
double free.  This patch prevents the Address Sanitizer error, allowing
tests to be run with Address Sanitizer enabled.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13927

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon May  6 08:55:22 UTC 2019 on sn-devel-184

(cherry picked from commit 193b4446)
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