tools: fix crash in virt-login-shell if config doesn't exist
tools: fix crash in virt-login-shell if config doesn't exist

If the 'allowed_users' config setting in virt-login-shell.conf
does not exist, we dereference a NULL pointer resulting in a
crash. We should check for this case and thus ensure the user
is denied access gracefully.

Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

libvirt/libvirt,jardasgit/libvirt,jardasgit/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,olafhering/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,crobinso/libvirt,libvirt/libvirt,jfehlig/libvirt,andreabolognani/libvirt,olafhering/libvirt,libvirt/libvirt,fabianfreyer/libvirt,zippy2/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,fabianfreyer/libvirt,olafhering/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,libvirt/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,andreabolognani/libvirt,zippy2/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,crobinso/libvirt,jfehlig/libvirt,jardasgit/libvirt,jardasgit/libvirt