sh: Avoid referencing uninitialized memory in alias

If run as
alias ''
uninitialized memory could be referenced.

This is based on a fix from NetBSD. For more information, refer to
https://github.com/NetBSD/src/commit/10cfed82c28 .

Obtained from: NetBSD (Robert Elz <kre@netbsd.org>, 10cfed82c28)
MFC after: 3 days

Reported by: mckusick, Robert Elz <kre@netbsd.org>
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D50364