MFC r242766: sh: Fix two issues when an alias is redefined:

MFC r242766: sh: Fix two issues when an alias is redefined:

 * The last character is not displayed.
 * If the alias ends with itself (as a word), an infinite memory-eating loop
   occurs.

If an alias is defined initially, a space is appended to avoid recursion but
this did not happen when an alias was later modified.

PR:		bin/173418
Submitted by:	Daniel F.

git-svn-id: svn://svn.freebsd.org/base/stable/9@243402 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f