tests: Fix runtime error in test-authz-pam

A test with sanitizers on macOS shows this error:

    authz/pamacct.c:50:25: runtime error: null pointer passed as argument 1, which is declared to never be null
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/security/pam_appl.h:56:2: note: nonnull attribute specified here

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>