sh(1): initialize smark to zero in main()

As popstackmark may be called on this without pushstackmark having
been called, we need to initialize it so that we don't get a bogus
comparison inside popstackmark, which would have resulted in a
NULL pointer dereference.

MFC After: 3 days
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40413