sh: Avoid referencing uninitialized memory in alias

[jrm]

Summary

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

This is based on a fix from NetBSD. For more information, refer to the
commit log in
https://github.com/NetBSD/src/commit/10cfed82c282ff96a5a8818a9ce9b0e7444d418d

Obtained from: NetBSD (kre)
MFC after: 3 days

[emaste]

Heh,

$ /bin/sh
$ alias '' ==
$ alias
Bus error (core dumped)
$ 

[emaste]

bin/sh/alias.c

208

n is by definition an empty string, so the message would be alias: : not found which is confusing

[jrm]

Improve error message