libss: initialize function pointer for readline_shutdown()

Clear the function pointer for readline_shutdown() so that if libss is
linking against a readline library which doesn't supply a
readline_shutdown() function, ss_delete_invocation() won't seg fault.

Thanks to Solar Designer <solar@openwall.com> for reporting this
problem to me.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
1 file changed